They are a bit like building blocks. Genes and cancer. There are many different types of cells in the body which do different jobs, but they are basically similar. They all have a centre called a nucleus. Inside the nucleus are the genes.
Genes are really bits of code.
The information they carry can be switched on or off. The genes control the cell. They decide when it will reproduce, what it does and even when it will die. Normally the genes make sure that cells grow and reproduce in an orderly and controlled way. If the system goes wrong for any reason, the usual result is that the cell dies.