Lemmings will fall through a trap door into each level. Your job is to get them safely to the exit by using the Lemming skills at the bottom of the play window. Simply click on a skill icon to use that skill. Some levels stretch beyond the limits of the window, so there will be scroll bars in those circumstances.
The game is a lot of fun since it presents a new puzzle with each level that must be figured out by planning ahead if all the Lemmings are to make it to safety. The author plans to implement 30 levels for each of the 4 increasingly difficult game categories: Fun, Tricky, Taxing and Mayhem; however, there are currently only 10 levels available for each.
The full version of the game uses graphics, sound and music from the original, though the author has not received permission to use them. My guess is you should get a chance to play it in case it disappears into the ether. You can even download the source code for the game (minus, of course, the copyrighted game assets) from the author's site, as well as other goodies. So what are you waiting for?