Turtle v. Tortoise

I always used the words [fairly] interchangeably. but there has to be a distinctish difference...

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. So what the crap is a terrapin? A terrapin is a type of turtle belonging to the emydidae family that lives in brackish or fresh water. Brackish water has a higher salt content than freshwater, but a lower concentration than seawater. The terrapin turtle is often compared to the sea turtle -- though they do not belong to the same family -- because of its webbed feet and relatively thin shells. Thank you wisegeek.com