deMOLEstration in Off Topic
If you keep track of the player's speed in separate x and y components, gravity is a constant acceleration downwards. Every tick gravity decreases your y-velocity by a constant amount. Jumping increasing your y-velocity by a large amount (over several ticks, if you'd like to allow for variable-height jumps).
You can see a simple example of this type of movement here: http://www.byond.com/developer/Forum_account/PixelMovement