First and foremost they could be categorised as turtles and tortoises. The following answer amply answers your question which I have picked up from Yahoo Answers:
Tortoises live on land. They usually have high, domed shells (though there are exceptions, like the pancake tortoise). They have thick, pillar-like legs to support their weight, and feet with stumpy claws. Here is an image of a leopard tortoise:
http://www.mccullagh.org/db9/10d-17/leop...

Turtles live in the ocean. They have low, flattened shells to make their shape more hydrodynamic. They have flippers instead of legs, the front pair of which are the largest and are used to propel them through the water. These cannot support their weight on land - when females come ashore to lay their eggs, they must drag themselves along with their front flippers (males normally do not come ashore). Here is an image of an olive ridley turtle:
http://www.nmfs.noaa.gov/pr/images/turtl...

There are also terrapins, which are semi-aquat ...more

Sea tortoise can swim...in land tortoise can walk. How amazing naa son ha ha ha ha ha ha ha ha

Tortoises or land turtles are land-dwelling reptiles of the family of Testudinidae, order Testudines. Sea Turtles (superfamily Chelonioidea) are turtles found in all the world\'s oceans except the Arctic Ocean. There are seven living species of sea turtles: flatback, green sea turtle, Hawksbill, Kemp\'s Ridley, Leatherback, Loggerhead and Olive Ridley. The East Pacific subpopulation of the green turtle was previously classified as a separate species, the black turtle, but DNA evidence indicates that it is not evolutionarily distinct from the green turtle.

Both are reptiles but most sea turtles do not leave the sea, unless it is nesting time.

Sea tortoise grows huge and survive for 400 years while land does not survune long and die fo some accident or become prey