The baseplate is usually held onto the deck using skateboard hardware (nuts and bolts, usually specific sizes and lengths). Sometimes, the baseplate will have a riser or shock pad sandwiched in between itself and the deck.
The reason for shock pads is to keep the deck from developing stress cracks. A lot of skaters don't use shock pads though - especially since if you are a very aggressive skater, you will likely break your board long before stress cracks are a problem. However, if you are a more casual skater, then shock pads might be a great idea for you.
Risers add a little height to the skateboard, which can be important if you are using larger wheels. Large wheels without risers can result in "wheel bite", where you go for a deep turn and the bottom of the deck touches the wheel, stopping that wheel instantly. This can end in some gnarly wrecks! But, if you have smaller wheels and your trucks aren't too loose, this shouldn't be a problem.
