The human body is made up of hundreds of different types of cell, all of which behave differently.
A cell in the kidney, although it contains the same genetic information as a brain cell, performs a completely separate role.
Cancer happens when a tiny part of the cell's mechanism goes wrong.
What is a cancer cell?
And just as there are hundreds of types of cell, there are hundreds of types of cancer, few of which can be treated in the same way.
Every cell's life is mapped out in advance by coded instructions, called genes, held in its nucleus.
These tell it how to behave, when to reproduce by dividing - and when to die.
When the instructions relating to cell multiplication and dying are wrong, the cell may start dividing uncontrollably, and not die when it should.
In addition, the cancer may not follow the usual instructions that keep cells spaced out properly.
Every time the cell divides, the "bad" instruction is reproduced, so the out-of-control multiplication carries on.
As these cells can be multiplying more rapidly than healthy cells, the cancer cells can form a growing lump in the body called a tumour or a lesion.
As this gets larger, it can even grow its own vessels to keep it supplied with blood.
A benign, or non-cancerous tumour shares this uncontrolled growth, but will not generally invade neighbouring tissues and damage them.
Tumours which do this are "malignant", or "cancerous".
The type of cell in which the cancer starts will generally determine the speed at which it grows, and its resistance to treatment, although there are many variations.
Cancers harm health in a number of ways. The very size of the tumour can interfere with nearby organs, or ducts which carry important chemicals, causing pain or other symptoms.
For example, a tumour on the pancreas can grow to block the bile duct, leading to the patient developing obstructive jaundice.
And a brain tumour can push on important parts of the brain, causing blackouts, fits and other problems.
Even benign tumours can cause these problems if located in the wrong place.
When a cancer invade nearby tissues, they can cause bleeding from damaged blood vessels, and stop the organ which they are invading from working properly.
What happens if it spreads?
As a tumour grows, cells can break off and start growing on adjacent tissues and organs.
For example, if a bowel cancer has spread through the wall of the bowel itself, it can start growing on the bladder.
Cells can also enter the bloodstream and travel to distant organs, such as the lungs or brain.
The technical term for this is "metastasis".
When new tumours form on distant organs, they behave like the original tumour - so a bowel cancer cell growing in the lung will be lung cancer.
Once other organs are involved, then any symptoms of the cancer can get worse.
However, it may be some time before a growing cancer in certain parts of the body produces symptoms that the patient can notice.
Once a cancer has started to spread beyond its original site, then the chances of a cure often begin to fall, as it becomes more difficult to treat.
How is it treated?
There are three principal ways of treating cancer.
The first is surgery, normally an operation to remove the cancerous growth, and, depending on its type, nearby tissues and organs.
A cancer patient may first undergo a minor operation called a biopsy to take a small sample of the cancer for analysis.
The surgeon will try to remove as much of the cancer as possible, but sometimes extra treatment will be needed.
This could either take the form of radiotherapy or chemotherapy, or a combination of treatments.
What is chemotherapy?
Chemotherapy is drug treatment which is used to try and kill cancer cells or stop them spreading.
Different cancer cells respond to different drugs, so not all chemotherapy is the same.
Sometimes as many as eight different drugs is employed to get the best effect, and doctors are constantly trying out new combinations to improve treatment.
Chemotherapy is often associated with debillitating side effects, but many types of modern chemotherapy cause only mild problems.
Who gets chemotherapy?
Because chemotherapy drugs are usually injected into the blood, they travel around the body and can attack cancer cells regardless of where they find them.
For this reason, doctors will use them when they think there might be cancer cells in more than one part of the body.
If some cancers have been growing for a while undetected, bits of them can break away from the main tumour and travel to either nearby tissue, or to more distant organs like the liver and lungs - and start growing there.
A surgeon can only cut out the a main cancer tumour, and nearby tissues which may be involved.
Radiotherapy, which uses radiation to destroy cancer cells, can only be given to small areas of the body or it will cause damage to too many healthy cells.
Often, after an operation to remove cancer, chemotherapy will be given to "mop up" any remaining cells.
Some cancers, such as leukaemia, need chemotherapy because they involve cells which are found throughout the body.
Chemotherapy can be given to shrink a tumour to make it easier for the surgeon to remove.
It can also ease the symptoms of patients whose cancer is not curable.
How does it work?
Chemotherapy, in its traditional sense, is a chemical which is poisonous to cancer cells and kills them.
This is called a cytotoxic chemical - one very early chemotherapy was produced from mustard gas, which was used as chemical weaponry during the First World War.
However, anything which is poisonous to cancer cells may also be poisonous to the body's healthy cells, which it needs to survive.
The trick with chemotherapy is to find a chemical which kills as many cancer cells as possible, and as few healthy cells.
Doctors have been getting increasingly successful at developing such chemicals, by spotting the differences between the cancer cells and neighbouring normal cells, and exploiting them.
The principle difference between many cancer cells and normal cells is the speed at which they reproduce, or divide.
Cancers tend to be dividing and growing faster than other cells in the body - which is why lumps or tumours sometimes appear.
Other cancer cells may become more or less active in response to natural chemicals called hormones produced by the body.
Some chemotherapy harnesses this reaction to control the growth of the cancer cells, so rather than poisoning the cells, they starve them of something they need to grow and multiply.
Cancer cells are not attacked by the body's own immune defence system because the immune system does not recognise them as foreign.
Some chemotherapies try to programme the immune system to see the cancer cells as foreign so they can be attacked and destroyed.
How is it given?
Often, chemotherapy is delivered by injection into a blood vein.
In many cases a saline drip will be set up to dilute the drug as it enters the body. This stops it harming the vein because it is so concentrated.
If the patient needs frequent doses of different drugs, to avoid the discomfort of having to have separate injections every time by having a tube kept in the vein which attaches to the syringe. This line is often left in a vein in the chest - one type is called a "Hickman line".
Sometimes, a concentrated dose of chemotherapy is needed on a particular part of the body, and side effects can be lessened by injecting it directly onto the cancerous area.
For example, for some bladder cancers, the drug can be pumped into the bladder so it works directly on the tissue involved.
How long chemotherapy courses last varies between different cancer types, with some being given intensively over a fortnight, normally in hospital, and some over a period of months.
What about side-effects?
Because some chemotherapy targets fast-growing, or fast-dividing cells, it is more likely to harm similar cells in the body.
These include the cells in the hair follicles, which is why cancer treatment is often associated with hair loss, although hair does regrow once treatment has ended.
Other fast-dividing cells can be found in the stomach and bowel lining, which leads to nausea and diarrhoea.
There are, however, drugs which help control this, and timing meals to avoid having a full stomach when the drugs take effect can also help in some cases.
Other types of normal cell that can suffer are the blood cells.
Red cells are important to carry oxygen to keep other cells alive. Other blood cells help stave off infection.
As a result, chemotherapy patients may be more prone to infections, and find them harder to fight off.
Hospitals will take regular blood tests to monitor the levels of different cells. This is called a full blood count.
Generally, patients on chemotherapy can be expected to have less energy than usual, or perhaps even feel extreme fatigue.
Chemotherapy can also affect the fertility of both men and women, and both can now freeze their sperm and eggs with the hope of using them to produce a test-tube baby.
What is radiotherapy?
It has long been known that radiation can damage human cells - and radiotherapy harnesses that power to treat cancers.
Ionizing radiation - similar to x-rays - can penetrate tissue, and alter the part of the cell which regulates its growth and reproduction.
Healthy cells can recover from this damage, while cancer cells cannot.
There are two types of radiotherapy - delivered from outside the body by a machine, and using radioactive implants placed inside the body.
Researchers are working to increase the effectiveness of radiotherapy by targeting the beam of energy more precisely, and making the cancer cells more sensitive to it.
Who gets radiotherapy?
The principal use of radiotherapy is to tackle solid tumours found in just one location, for example skin, brain, breast or uterine cancers.
Sometimes doctors will use the treatment to shrink a tumour so that a subsequent operation will be more effective.
In some cases, for example in invasive bladder cancer, radiotherapy is considered as the first option, as an alternative to surgery that would have permanent effects on the lifestyle of the patient.
But although radiotherapy alone can cure many cancers, in other cases the radiotherapy is given after surgery over the surrounding area to "mop up" any remaining cells which have spread from the original cancer site.
If there is a suspicion or firm evidence that cells could have spread further afield, then chemotherapy may be the preferred option.
What form does treatment take?
If the radiotherapy is delivered by a machine, then the patient will normally be given repeated treatments over a brief period.
Although treatment timing varies depending on the type of cancer, its location, size, and the dose chosen by the doctors, it is not unusual to be given treatments every day for a few weeks.
If the therapy involves inserting a radioactive implant near a tumour, then a brief hospital stay is normally needed.
What about side-effects?
Although the treatment itself is painless at the time, the cumulative effect of many sessions does produce side effects.
The radiation can produce a sunburn-like effect on the skin as it passes through. The extent of this depends on the number and intensity of treatments.
There can be hair loss in the area being treated - which is usually temporary.
The treatment can also leave the patient feeling fatigued and generally lethargic.
Are there long-term risks?
Ionizing radiation produces changes within the genetic structure of the body's cells, and there is a small risk that an increased radiation dose leads to changes in healthy cells which can cause cancer.
New equipment and techniques help to reduce the risk of side effects by targeting doses more accurately on the cancer, thus lowering the dose received by critical organs and tissue around the treatment area, while allowing the malignant tissue itself to be zapped by higher doses than before.
Older radiotherapy equipment tended to target a larger area, so healthy tissue as well as cancerous tissue would be irradiated, increasing the risk of side effects.
The UK government is now investing in the latest machines.
The risks of medical radiation exposure are miniscule when compared to the risks to the patient's health of not having the treatment.
What's likely to happen in the future?
Researchers are fine-tuning radiotherapy to improve the outcomes for patients.
The main efforts focus on delivering a more powerful radiotherapy beam accurately to smaller and smaller targets.
Some drugs seem to make cancer cells more vulnerable to radiation, which means less powerful radiotherapy, or fewer sessions, are needed.
Another field of research is looking at heating cells in a specific area to make them more sensitive to radiotherapy.