A psybuck is the money at psypoke. You can get more by1. Taking part in competitions, 2.If you open a shop you can sell items such as sigs and avatars etc. ,3.The administrators may reward you with psybucks for doing something like making a guide etc.
As you know you get 50 psybucks for joining. Also some people call them PB for short so if you see that you'll know what it means.
There may be a few other uses that I am unaware of or jave forgotten to mention