Lab mice are much like regular mice, but are used as tools in scientific study and experiementation. The stereotypical lab mouse is an albino, with white fur and skin and red eyes. Some have been specially bred or genetically engineered to exhibit certain traits which are then studied.
There is considerable controversy over whether this practice is ethical, but it goes on nonetheless.
Lab mice at UC Davis are occasionally sent to other universities for study there. Few transport companies will accept live animals; DHL is one of the few that does. One of the worst parts of being an employee in the Central Stores and Receiving department at UC Davis is periodically having to listen to 100+ tiny lab mice skitter and scratch around in their cardboard shipping boxes until the transport folk arrive — like nails on a chalkboard, only a thousand times worse.
It should be noted that lab mice (and their equivelent for sale as snake food) tend to make poor pets for several reasons. They tend to be bred to purpose (and inbred), and as a result display neurotic behaviors. They also get along poorly with mice that have a better breeding heritage (generally referred to as "fancy mice").