Rats
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, Rattus rattus, and the brown rat, Rattus norvegicus. Many members of other rodent genera and families are also called rats and share many characteristics with true rats.
Rats are typically distinguished from mice by their size; rats are generally large muroid rodents, while mice are generally small muroid rodents. The muroid family is very large and complex, and the common terms rat and mouse are not taxonomically specific. Generally, when someone discovers a large muroid, its common name includes the term rat, while if it is small, the name includes the term mouse - scientifically, the terms are not confined to members of the Rattus and Mus genera. Compare the taxonomic classification of the Pack rat and Cotton mouse.
Dangers
Rats often cause electrical fires in buildings by gnawing through plastic electrical junction boxes. Rats must constantly gnaw on hard objects to cut back their constantly growing incisor teeth.
In addition, they live in the most unsanitary places, and are carriers of serious health risks to humans from their droppings and constant incontinence (they use urine trails to find their way in the dark).