I'm quoting the website wisegeek.com:
A division of reptiles, called chelonians, includes turtles, tortoises, and terrapins. For the most part, the difference between a turtle and tortoise is more of a rough semantic category than a strict taxonomic separation. Colloquially, both biologists and lay people use the word, "turtle" to mean all chelonians. In general, turtles live in or near the water and have adapted to swim by holding their breath underwater. Tortoises live primarily in arid regions, built for storing their own water supply and walking on sandy ground.
In a biological respect, a tortoise is a kind of a turtle, but not all turtles are tortoises. Tortoises occupy their own taxonomic family, called testudinidae. All types of land and aquatic turtles come in a wide range of sizes, colors, and shapes. Sometimes the name "terrapin" refers to those animals that fall somewhere between a turtle and tortoise, because they live in swampy areas or begin life underwater and eventually move to dry land.
Turtles may live in freshwater, the ocean, or brackish ponds and marshland. Their front feet might be fins or merely webbed toes with streamlined back feet to help them swim. Turtles have flatter backs than tortoises, and may spend all or part of their lives underwater. They mate and lay eggs underwater or on the shore. Some turtles sun themselves on logs, rocks, or sandy banks. During cold weather, they burrow in mud and go into torpor, a state similar to hibernation. Sea turtles migrate great distances. They are more often omnivorous, eating plants, insects, and fish.
Tortoises live entirely above water, only wading into streams to clean themselves or to drink. In fact, they could drown in deep or swift current. Their feet are hard, scaly, and nubby so it can crawl across sharp rocks and sand. Tortoises often have claws to dig burrows, which they occupy during hot, sunny weather or during sleep. Tortoises are mostly herbivorous, eating cactus, shrubs, and other plants that have a lot of moisture. They rarely migrate. Their shell forms a rounded dome, allowing the tortoise's limbs and head to withdraw for protection.
