The term longboard is the english word for "long skateboard". A longboard is between 90cm and 160cm long and therefore and has a longer wheelbase compared to a skateboard. Longboard wheels are made of polyurethane and softer and larger then skateboard wheels. With bigger and softer wheels bumps on the road can be absorbed better. Due to the larger wheelbase, it is possible to reach higher speeds than with a skateboard. The special longboard trucks ensure the required stability at higher speeds.
Longboards are mainly made to commute and as an alternative for your bicycle and rather less to do tricks.