There are 3 basic things you need to keep in mind when designing or folding paper airplanes. These things are Lift, Drag and Thrust.
Lift is what allows planes to fly through the air as easily as fish swim through the water. When a plane is moving through the air, air is moving along the surface of its wings. The angle of the wind moving against the wings surface will produce a force that will push the wing in a particular direction. If you've ever stuck your hand out of a moving car window and closed your fingers like you are about to judo-chop something then you have felt lift in action. The angle you hold your hand at will determine the direction your arm gets pushed. The same principles are true with paper airplanes.
Drag is the result of friction and turbulance between the air and a surface. When you are submersed in water and you try to swing your arms or legs as fast as you can you've probably noticed that you cannot move them as quickly as you would be able to in air. This extra effort that is needed is caused by drag. Though it is less cumbersome when applied to air than when applied to water, it is still there.
Thrust is the forward force that is applied, whether is comes from the engine of a jet or from your hand.
When you are file tuning your paper airplanes wings and body to hone its flight characteristics, remember that the two forces your are using are lift and drag. If you fold the back of your wings slightly up, you are goung to create lift by changing the angles of impact between the air and the wings. You will also be creating slightly more drag on the top-rear of the wings.
If you would like more information about aerodynamics, try these two sites:
Wikipedia's aerodynamics entry
PaperPlane.org's Aerodynamics section