A shortboard is a short surfboard. It is called
a shortboard because it is shorter than a longboard.
Modern shortboards usually have three fins, with a
large fin in the middle and two smaller fins
towards the rails.
Shortboards are more manoeuvereable than longboards,
so all the neat tricks you see the professional surfers
doing are done on shortboards.
People learning to surf will find a longboard
easier to start with, and then they can change to a
shortboard later.