The moles are looking for lawn larvae and bugs like crickets and grubs, earthworms, and other pests that take residence in your grass and it's roots. It may only take one or 2 moles to completely trash your yard. It'll look like little humps of dirt popping up everywhere, and you may never ever see the little ****. They live in their tunnels and they love it when you water your lawn.
Best way to get rid of them is do away with the food source and they will migrate to your neighbor's yard. You can set mole traps if you want, or poison. But that's dangerous to pets and kids.
Here is a link I found to be most informative. Home depot and Lowes carry a wide variety of pest control products and if it gets too serious....
call in the pros.
http://www.pestproducts.com/moles.htm