What is a reed valve system and what does it do? Furthermore... what kind of situation or quad is it usually used for?

Just curious.

 

All two-strokes use some kind of reed valve system. In a nut shell a four-stroke uses a valve to let in and out gases, and fires on the compression stroke.
A two-stroke relies on a reed valve to keep air and fuel mixture to stay in the intake portion of the jug. Then when the piston comes down and past the transfer port windows the fresh air fuel mixture pushes the burned exhaust gases through the exhaust port. When the piston travels up for the compression stroke at the same time the reed valve opens and lets fresh air fuel mixture once again.

I think this is right!! Close enough!!