Biplanes are airplanes with two full sets of wings with a cockpit for the pilot sandwiched between the two. Before the jet engine was introduced these airplanes offered maneuvering capabilities superior to any other planes at the time. Building a paper biplane is no more difficult than making a traditional paper airplane and like the planes they are based on they offer better stability and performance.
Fold the paper in half 'long way' so you are left with a long, thin rectangle. Open this and fold the top edges down to the crease so the paper looks almost like a tall house.
Fold the paper in half along the center crease. Fold one of the flat edges down to the outside of the center crease to create a wing. Flip the plane over and repeat to create the second wing.
Cut a 3" x 8.5" strip of paper from the second sheet. Fold this in half lengthwise and then open it back up. Mark a point 1 inch on either side of this center fold. Cut a short pyramid with a flat top by trimming away a diagonal line between these marks and the far corners; these are your secondary wings. Fold the paper in half again along the central fold, and fold the wings back the opposite direction.
Slide the bottom of the plane into the divot of the secondary wings created by the center fold. Use a piece of scotch tape on either side of the airplane's body to adhere these to the plane. Add another pice of scotch tape to top of the paper airplane to keep the two side of the plane from spreading apart during flight. Give your airplane a toss to test its flight capabilities. If it does keep aloft try trimming a bit of paper from the end of you secondary wings, making sure to remove the same amount from both sides.
Try to use as little scotch tape as possible to avoid weighing down the plane unnecessarily. Also be sure to apply it to either side symmetrically, as too much tape on one side of the plane will cause it to spiral in that direction during flight.