The main differences between a gerbil and a mouse are their size, appearance, behavior, and habitat. Gerbils are typically larger than mice, have a longer tail, and are known for their friendly and social nature. Mice, on the other hand, are smaller in size, have a shorter tail, and can be more skittish and less social than gerbils. Additionally, gerbils are desert animals and require a sand bath for grooming, while mice are more adaptable to various environments.