A tortoise is a very specific species of reptile that is a land dweller. Some tortoises get to be so big that a small child can ride on them without harm to the reptile. Turtles live in or around water though they breathe air and lay their eggs on land. A turtle can not survive in barren areas, such as the Karoo Desert, where some tortoises prosper because, even though they go about on land, they must be in or near water, whereas tortoises are not restricted to the proximity of water any more than other land animals are.
According to my resources, the difference is not a scientific one, but is used in common speech to express habitat differences, and which is which depends on where you are in the world- Australia, for instance, uses the terms differently than the US. In the US, tortoises are the members of the chelonia that live on land, turtles live in the water, and terrapins do both - they live along shorelines and spend time both on shore and in the water.
In Australia, only salt water turtles are called "turtles", and everything else is called a tortoise. So a sea turtle there is a turtle, but a snapping turtle would be a tortoise down under.
The major difference to me is that turtles are aquatic and tortoises are terrestrial. Turtles can come on land, but they live most of their lives in the water. Tortoises live their lives primarily on land. One physical difference that stems from this has to do with their feet. Tortoises have round feet while turtles have webbed feet.