Rats are various medium sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus Rattus, the most important of which to humans are the black rat ...
http://en.wikipedia.org/wiki/Rat

The brown rat, common rat, Hanover rat, Norway rat, Norwegian rat, or wharf rat (Rattus norvegicus) is one of the best known and most common rats, and also one of the largest.
http://en.wikipedia.org/wiki/Brown_Rat

RATS seeks to further the art and science of amateur radio.
http://www.rats.net/

What you need toknow about deciding on a rat as a pet - life span, size, and characteristics. ... Rat Facts. Average life span of 2-3 years. Length: body 9-11 inches; tail 7-9 ...
http://exoticpets.about.com/cs/careofrats/a/ratsaspets.htm

Articles about the behavior of rats, their biology and the sensory world they live in.
http://www.ratbehavior.org/