Mutations are random changes in DNA. Mutations can be caused by many factors including radiation, a mistake in DNA replication, carcinogens or even viruses. There are two main types of mutations that can occur. These include insertion mutations and deletion mutations.
Insertion mutations happen when an extra nucleotide is inserted into a set of nucleotides causing a shift of the nucleotides that follow it therefore changing the codons. This can cause the wrong amino acids to be coded for.
A deletion mutation, like its name suggests, is caused when a nucleotide is deleted from a set of nucleotides. This also causes a shift of the remaining nucleotides and affects the codons and the coding of amino acids.