The Pelican Gel Skins are small plastic-rubber ovals that stick on either side of your XBox 360 controller. If your hands get sweaty, they can help you hang on to the controller more easily.
The pads have a peel-off backing and then just stick with built-in glue to the sides of your controller. It's very easy to attach them. The pad itself is sort of "sticky" and holds well in your hand. There are small raised bumps on the pads to help further with the hand grip.
So the question becomes - is this really necessary? It's less than $10 for 4 skins (2 controllers' worth) so it's not a lot of money you're spending. The sides of the standard controller are quite slick and smooth. You probably don't care at all if you play Xuma for an hour or two. On the other hand if you get into a weekend-long Doom fragfest, with your heart pounding and sweat building up, it can definitely help to get a better grip on the controller.
One thing to keep in mind is that the grips are not "zero thickness". If your hands are on the small size, it might be that the current controller fits you perfectly - but that the controller plus grip is going to be a bit too bulky for you. Of course a lots of those sorts of things come down to what you're used to. After a few weeks with the grip, you might not mind that at all.
If you're a casual gamer, you probably won't need these gel pads. If you're a hard core gamer that plays multiple hours at a time, give them a try. If you really don't like them, you'll probably find a friend who will gladly take them off your hands.
Buy XBox 360 Gel Grips from Amazon.com