Although the word turtle is widely used to describe all members of the order Testudines, it is also common to see certain members described as terrapins, tortoises or sea turtles as well. Precisely how these alternative names are used, if at all, depends on the type of English being used.

• British English normally describes these reptiles as turtles if they live in the sea; terrapins if they live in fresh or brackish water; or tortoises if they live on land. However, there are exceptions to this where American or Australian common names are in wide use, as with the Fly River turtle.
• American English tends to use the word turtle for all freshwater species, as well as for certain land-dwelling species (e.g. box turtles). Oceanic species are usually referred to as sea turtles, and tortoise is restricted to members of the true tortoise family, Testudinidae. The name terrapin is typically reserved only for the brackish water diamondback terrapin, Malaclemys terrapin; the word terrapin being derived from the Algonquian word for this animal.
• Australian English uses turtle for both the marine and freshwater species but tortoise for the terrestrial species.

To avoid confusion, the word chelonian is popular among veterinarians, scientists, and conservationists working with these animals as a catch-all name for any member of the superorder Chelonia which includes all turtles, tortoises and terrapins living and extinct, as well as their immediate ancestors. It is based on the Ancient Greek word chelone, meaning tortoise.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Turtle, Tortoise