Rodents (Rodentia) are placental mammals. They contain 2,000-3000 species depending on the source, i.e. half of all recent mammal species. Rodents live on all continents and in all habitats, with the exception of the sea.
Rodents have two incisors[?] in the upper as well as in the lower jaw which grow continuously and must be kept worn down by gnawing; this is where they get their name, from Latin rodere, to gnaw. These teeth are used for cutting wood, biting through the skin of fruit, or for defence. Nearly all rodents feed on plants, but there are a few exceptions which eat insects or even fish.
The order Rodentia may be divided in suborders, superfamilies and families. This is a common classification scheme:
Recent work has suggested that the rodents may actually be biologically polyphyletic (i.e. have evolved more than once), in which case the group would have to be split up.