Are you one of the many who are confused with what is the difference between a turtle, tortoise and a terrapin? Your confusion is about to be solved. A division of reptiles, called chelonians, includes turtles, tortoises, and terrapins.
A turtle is any reptile that lives in a shell. A tortoise is a turtle that lives on the land but can enter the water for a drink, to cool off or to escape and evade predators. Turtles have flatter backs than tortoises, and may spend all or part of their lives underwater. A terrapin is a turtle that lives primarily in water, generally leaving only to lay eggs or to bask in the sun.
In a biological aspect, a tortoise is a kind of a turtle BUT not all turtles are tortoises. Tortoises belong to a specific taxonomic family, namely testudinidae. They live entirely on land and only take a dip into shallow water to clean themselves or to drink. As a matter of fact, they are not good swimmers like the turtles. They could drown in deep or swift current.
That should clear your confusion regarding turtles, tortoises and terrapins. They all make good pets.