The main difference is that a turtle is adapted for living in the water while a tortoise is adapted for life on land. You can see this in the fact that turtles have thinner more water-dynamic shells to aid in swimming. Tortoises have elephant-like feet for walking on land while turtles have webbed feet or flippers to aid in swimming. This is primarily a semantic difference though as despite them having differences turtles and tortoises both belong to the same family of animals we have only colloquially decided to use the word tortoise to describe land-dwelling members of the turtle family
