From ISOGG Wiki
A mutation is a change in a DNA sequence. Mutations can result from DNA copying mistakes made during cell division, exposure to ionizing radiation, exposure to chemicals called mutagens, or infection by viruses. Germ line mutations occur in the eggs and sperm and can be passed on to offspring, while somatic mutations occur in body cells and are not passed on.
This article uses material in the public domain from the Talking Glossary of Genetic Terms and is reproduced courtesy of the National Human Genome Research Institute.