The best-known rat species are the Black Rat (Rattus rattus) and the Brown Rat (Rattus norvegicus). The group is generally known as the Old World rats or true rats, and originated in Asia. Rats are bigger than most Old World mice, which are their relatives, but seldom weigh over 500 grams (1 lb) in the wild.

The term “rat” is also used in the names of other small mammals which are not true rats. Examples include the North American pack rats, a number of species loosely called kangaroo rats, and others. Rats such as the Bandicoot rat(Bandicota bengalensis) are murine rodents related to true rats, but are not members of the genus Rattus. Male rats are called bucks, unmated females are called does, pregnant or parent females are called dams, and infants are calledkittens or pups. A group of rats is either referred to as a pack or a mischief.

In some developed countries, many people keep domesticated rats as pets. These are of the species R. norvegicus, which originated in the grasslands of China and spread to Europe and eventually, in 1775, to the New World. Pet rats are Brown Rats descended from those bred for research, and are often called “fancy rats”, but are the same species as the common city “sewer” rat. Domesticated rats tend to be both more docile than their wild ancestors and more disease prone, presumably due to inbreeding.

The common species are opportunistic survivors and often live with and near humans, therefore they are known as commensals. They may cause substantial food losses, especially in developing countries. However, the widely distributed and problematic commensal species of rats are a minority in this diverse genus. Many species of rats are island endemics and some have become endangered due to habitat loss or competition with the Brown, Black or Polynesian rat.

Wild rats can carry many different “zoonotic” pathogens, such as e.g. Leptospira, Toxoplasma gondii and Campylobacter, and may transfer these across species, for example to humans. The Black Death is traditionally believed to have been caused by the micro-organism Yersinia pestis, carried by the Tropical Rat Flea (Xenopsylla cheopis) which preyed on Black Rat living in European cities during the epidemic outbreaks of the Middle Ages; these rats were used as transport hosts. Today, this cycle still exists in many countries of the world and plague outbreaks still occur every year. Beside transmitting zoonotic pathogens, rats are also linked to the spread of contagious animal pathogens that may result in livestock diseases such as Classical Swine Fever and Foot-and-mouth disease.

The normal lifespan of rats ranges from two to five years, and is typically three years.