A mutation is a change in the genetic composition of the DNA. It is often caused by an accidental insertion, deletion or swapping of a single base however whole genes can also move.
SNPs, or single nucleotide polymorphisms, are the most common type of mutations. They mostly occur in the non-coding region of DNA however they can occur in the coding region, within genes. This can sometimes be harmless, if that part of the DNA still codes for the same amino acid in a protein but if it codes for a different amino acid, it can change the shape and therefore the function of the protein. Sickle Cell anaemia is one of the most well known diseases caused by an SNP. SNP's are an example of a point mutation
Mutations can be caused by chemicals, radiation by natural causes.