Cells are always having a few mutations in their DNA, but there are systems in the cell that will recognize excess mutations and kill the cell before it can cause problems.

Things can really start to get out of hand when you get mutations in those proofreading systems themselves. Then you get a set of cells that can start mutating freely. Eventually natural selection takes over and the mutant cells keep accumulating more and more mutations that help them replicate and spread, leading to cancer.