Rats are various medium-sized, long-tailed rodents and are typically distinguished from mice by their size. Generally, when someone discovers a large muroid rodent, its common name includes the term rat, while if it is smaller, its name includes the term mouse. The common terms rat and mouse are not taxonimically specific. In other words, rat is not a scientific term; if it were, all rats would belong to the genus Rattus. The best-known rat species are the black rat and the brown rat. Rats are found in houses, garages and under soils and feed crops such as rice, wheat and maize. Rodents family include mice, rats, squirrels, beavers, hamsters, porcupines etc.

Male rats are called bucks; unmated females, does, pregnant or parent females, dams; and infants, kittens or pups. A group of them is referred to as a mischief. 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, some of which have become endangered due to habitat loss or competition with the brown, black, or Polynesian rat. Their average lifespan varies by species, but many only live about a year due to predation.

Wild rodents, including rats, can carry many different zoonotic pathogens, such as Leptospira, Toxoplasma gondii, and Campylobacter. A zoonotic disease linked to the rat is foot-and-mouth disease. They can carry various other kinds of diseases such as flu and viruses.