Until fairly recently homes were protected from most termites by treating the soil around the home's foundation with highly toxic and long-lasting insecticides such as chlordane. Even today soil-applied insecticides are still used but the insecticides now are less toxic and less persistent, and may be less effective.
Baits designed specifically for termites are now more widely used. Termite baits combine a low toxicity insecticide with an attractive (to termites!) food, usually some kind of cellulose. Bait stations, usually a plastic tube containing a bait that can be buried below the soil surface, are placed around the property where foraging termites find them and carry poisoned food back to the termite colony. Control is maintained over time with periodic replacement of the bait.
Termite baits are much less toxic and environmentally hazardous than old-style, soil applied insecticides. They are very effective when used properly and maintained over time. The main disadvantage, however, is they do require regular, long-term maintenance and for this reason can be expensive compared to a one time insecticide application.
There are several termite bait products available to homeowners as DIY projects however most are only available through pest control companies.
For much more information about termite baits, including current products, see the following article: Baits for Subterranean Termites.
I've also posted a series of articles about termites in general starting here including pictures of the different termite castes.