A skateboard is a wooden board, a pair of trucks and a set of wheels. But there are quite a few differences between the models.
A skateboard is primarily used for performing tricks. Narrow boards are preferred for street and wider boards for vertical and bowl. They are not intended for transport. It is possible but less comfortable because of the small and hard wheels.
Cruisers are made to move. They often have a flat surface but they exist with a raised tail. Cruisers are lighter than longboards and generally larger than skateboards. They allow rapid acceleration and provide balance and comfort.
Longboards are used for long distances but also for downhill races. Due to their size, longboards are not as responsive as others. Acceleration is slower, but the size of the board and wheels allows for higher speeds.