Rats and mice, together with other not-so-common mammals, are known as rodents.
Rats
The most common rat found in the UK is the brown rat. The species is believed to have its origins in the Far East but is now well-established throughout the United Kingdom in vast numbers. In fact, they outnumber humans.
Rats vary in size and weight with fully grown males growing up to 25 cm in length and can weigh up to 500 grammes. Females of the species are generally smaller than the males.
As well as causing considerable damage in the home and workplace, rats can carry a wide range of diseases that can result in serious health issues such as Weil's disease
Mice grow up to 20 cm in length (including the tail) and tend to migrate towards areas of warmth during cold periods. Due to their size and extreme flexibility, they can enter a building through the smallest of spaces. Primarily nocturnal animals, mice compensate for their poor eyesight with a keen sense of hearing, and rely especially on their sense of smell to locate food and avoid predators.
Rodents on Wikipedia