A tortoise is a reptile with a hard, rounded shell. It's a type of turtle that lives on land all the time — so bring your pet tortoise in the sandbox with you, but not in the pool!
A tortoise is a turtle, but a turtle isn't necessarily a tortoise! While most turtles are excellent swimmers and spend much of their time in the water, tortoises are full-time land dwellers. Their shells are more rounded than those of the typical turtle, and they don't have webbed feet. These are adaptations that help tortoises thrive on land, although they are notoriously slow (think "The Tortoise and the Hare").