What is the difference between a tortoise, a terrapin, and a sea turtle?

The difference is mainly in the habitat they are adapted for. Sea turtles are usually completely aquatic (except to lay eggs) and they are omnivores. Tortoises are usually land-dwelling and are mostly herbivores. Terrapins are semi-aquatic. They live in swamps and near brackish waters. Half their time is on land and half of the time they are in water.