Have a look at this topic to identify what species they are
http/www.fishforums.net/index.php?showtopic=75554
this will also tell you if the snails will do any damage to plants etc, the vast majority of them are harmless but they can take over the tank.
the single most important thing to remember with snail population control is that the population will only grow if there is food for them to eat. so in line with this the single most common reason people have a snail problem is that they are overfeeding and there's lots of food left over so the population of snails grows.
if you're struggling with snails then cut back on your feeding and the population will reduce in line with this.
if you want to remove snails from the tank some people suggest putting a slice of cucmber or a lettuce leaf into the tank and leaving it overnight, in the morning it'll be covered with snails and you can then remove it and the snails with it. it doesn't always work but it's certainly worth a try. if you've got a lot of snails you can do this a few nights running to make a dent into them.
if you have fish in the tank then you can crush the snails against the glass with your finger and the fish will go crazy for a nice meaty treat of snail!
steer clear of shop bought snail remedies, most of them just knock the snails out and you have to pick them out manually (why bothed knocking them out, you can just pick them out manually anyway for free?!) and the ones that do kill the snails contain copper which will also kill off any other inverts in the tank such as shrimp.