Certain powers grant the ability to fly. Control while flying is the basically the same as on the ground, with a couple alterations. In addition to the ground movement controls (W: forward, S: backward, A: strafe left, D: strafe right, Q: turn left, E: turn right), flight adds two more. Holding spacebar will move you straight up and X will move you down. These directions are relative to your camera view, so if you have angled your PoV to be looking down, holding spacebar will actually send you "forward" along the ground.
These powers grant the ability to fly:
There are also temporary powers which grant flight:
Finally, there are powers which enhance existing flight speed without actually giving flight ability.