Rats are various medium sized rodents. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat, Rattus rattus, and the brown rat, R. norvegicus. Many members of other rodent genera and families are also called rats and share many characteristics with true rats. A rat has an average life span of 2-3 years.
Rats are distinguished from mice by their size; rats generally have bodies longer than 12 cm (5 inches). Squirrels of most species are about the same size as rats but are members of their own family, Sciuridae, and are usually more specialized than rats.