The premise is simple:
You must avoid the relentlessly marching bars and you must not touch the floor or the ceiling.
The longer you stay alive, the higher your score.
Of course, the longer you last, the faster the bars will move up and down ...
you didn't expect it to be that easy did you?
You can also build points by trying for bonuses:
Press the mouse button, space bar or up arrow to thrust. Release to let gravity drop you.
Pressing 'P' pauses the game - press it again or click the mouse to continue.
Download the source code. The source is split into separate class files, so you won't be able to drop it straight into PDE. You'll need to view it with Eclipse or a plain old text editor, such as the rather spiffy Editplus.