What is a mutation?

The DNA is composed of units – the genes -, which contain (codify) the information that determines how cells work. Genes do this by producing functional particles – proteins – that coordinate all the cell activity.

A mutation is a change in the structure of the gene. Most of the time these changes have no obvious effect on the function of the gene (neutral mutations). And sometimes they can even be beneficial, giving the cell a functional advantage (these are the mutations driving evolution).

Harmful mutations, though, can make the cell malfunction, increasing the chances of diseases such as cancer. 

Contrary to common belief most mutations are not harmful.

 To prevent problems, the cell has several  DNA repair mechanisms  to fix these errors/mutations.