In biology mutation is any physical change in the genetic material of an organism. In most cases this is either the DNA or RNA in the cell nucleus. In multicellular organisms there are two primary classes of mutation, germline mutations and somatic mutations. Germline mutations are those changes that can be passed down to offspring, while somatic mutations are mutations that only alter genetic material in the mutated organism. There is some evidence that changes outside of the cell's genetic material, such as the cytoplasm, proteins, or the cell membrane can also be inherited.
Mutations can be caused by internal or external factors. Common external factors include ultraviolet radiation, chemical mutagens, or parasitic organisms (such as viruses or bacteria). Most internal causes of mutations stem from errors in reproduction of genetic material. Most of these errors are corrected by error-correcting ribosomes.
Some organisms will respond to harsh environments by increasing the rate of mutations. This is known as hypermutation and is hypothesized to aid organisms by creating wider variation in the gene pool of the population, increasing the chances that at least some decedents might survive under harsh conditions.
Biology (7th Edition). Neil A. Campbell,Jane B. Reece. http://www.amazon.com/Biology-7th-Neil-Campbell/dp/080537146X.