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.