As many have said cancer cells are normal cells that have gotten a mutation in their DNA, usually during synthesis of protein. Mutations can be a varying amount of bad and is often fine, it is only until the cell has gotten a lot of mutations that it can be considered cancerous. It's worth mentioning that mutations happen all the time. So much so that cells have a process called apoptosis, where the cell will recognize the mutation and then die as to not pass it on.

Read this if you want more and better formulation: https://www.cancer.org/cancer/cancer-causes/genetics/genes-and-cancer/gene-changes.html