What do the terms rat and mouse mean?

Rat and mouse are actually not scientific classifications. These words are common names for rodents that look alike to the casual eye.

Rat is used to describe medium-sized rodents with long thin tails. There are many species of rodent that are called rats — kangaroo rats, cotton rats, Norway rats, black rats and many others. These different rodent species may not be closely related to each other at all!

Mouse is used to describe tiny, sparrow-sized rodents with long thin tails. As with rats, there are many species of rodents called mice which may or may not be closely related to each other: house mice, field mice, deer mice, smoky mice, spiny mice, and dormice are all called mice.

So, which rats and mice are you talking about? Generally, people are referring to the domestic or pest rats and mice, which means Norway rats (Rattus norvegicus), black rats (Rattus rattus), and house mice (Mus musculus)

How can I tell Norway rats and house mice apart?
Adult rats and mice

Adult mice are much smaller than adult rats (Fig. 1). Adult mice weigh about 30 grams, and fancy mice tip the scales at about 50 grams. Adult mice have bodies that are 3-4 inches long with 3-4 inch tails.
Adult rats are far heavier and longer: they can weigh ten times as much, averaging 350-450 grams for females and 450-650 for males (with an overall range of 200-800 grams). They have 9-11 inch long bodies and 7-9 inch tails .