Cancer represents a big range of conditions. A cancer occurs when the normal controls on how a cell works are lost.
We are all made up of a huge number of cells that all have particular jobs to do, from making blood cells to building muscles and bones. How long these cells live for and how many copies they made of themselves is normally very closely controlled. If damage occurs in the genes, the blueprint of how the cell works, then they can start behaving in an abnormal way. For example, start producing lots of copies of themselves and not dying when it is normally time for them to be replaced. This means lots of abnormal cells can build up in that person’s body and start to affect how the body works.
What problems this causes and how it affects that person e.g. what symptoms they experience, depends on where these abnormal cells build up and what their job was before they became abnormal. Knowing which type of cancer we are dealing with has a big impact on the type of treatment that is needed.