Your cams spin. When they spin, they open and close your valves. They are driven by the timing belt, which comes off of the crankshaft pulley. Adjustable cam gears let you adjust--either advance or retard--the timing of each cam in the case of a DOHC motor. What this allows you to do is both play with the overlap (the amount of time both the intake and exhaust valves are open) and the overall relationship between the opening of the valves compared to when the pistons are at TDC.

In simpler terms, you don't really need them unless you have aftermarket cams which need to be tuned with your stock motor. They're a tuning tool, that's about it.