Ditto duckliondog.

I'll add:

You can get "liners" which go between tire and tube. I think Tufo makes some but I'm not sure.

Schwalbe makes tires with built in liners - their "Marathon Plus" being one. You can ride over a thumb tack and it won't puncture the tube. Schwalbe is also either working on or have available other types with the same built in liner.

Slightly higher rolling resistance than a non-lined tire but if you travel in areas where punctures are common, this type of tire is really a necessity. (i've heard of people getting two punctures *an hour* with regular tires in rough equatorial areas...I'd rather deal with higher rolling resistance than be flatting that often)