We have over 25,000 genes in our body. Genes have DNA whose job is to follow instructions to create proteins in our body. The proteins are designated to perform specific functions. They can be switched on and off. 
In Cancer the cells mutate causing permanent change in our DNA by substitution and deletion. Even though we have a genome surveillance system that can identify and sometimes repair damaged DNA it doesn't catch all the damages. These cells receive wrong or corrupt instructions and develop into  cancer cells. These cells replicate causing more cancer cells.  Check out this great Ted Talk on this process. Click the button below or check out the Image at the bottom of the page.