looks like a ramshorn snail to me, but it also could potentially be an apple snail. Do you have any more pictures of them? That would help a lot.
Ramshorn snails are seen as either a benefit or a nuisance, depending on your experience with them. They keep the tank and plants algae free but breed like crazy. Apple snails get gigantic (I believe I have some pictures of one of mine in my albums), they aren't very effective algae eaters, they WILL eat delicate plants when they run out of other food sources, and if you have multiples they will also breed like crazy. They're more for show because of their size and coloration.