But i still don't understand what I can do with one of these.
I can't get my head around the "gain" aspect of the transistor.
Is it simply a switch
If I send the arduino's 40 mA current I could drive a light / motor with a maximum of 40*60 = 2400 mA
IC Collector Current - Continuous 200 mA
it merely allows the passage of a certain current / voltage in excess to the one it sent to the base?
Think of it as a current controlled tap opening or closing a valve allowing more or less water (current) to flow.
With that number it can be pretty easy to figure out the current flowing in your load circuit-