The distinction between a turtle, tortoise or terrapin comes from what habitat they are adapted to live in.
Marine-based turtles spend most of their lives at sea, only returning to land to lay eggs. Their shells are flat and streamlined to help reduce drag while swimming. Unlike land-based tortoises who have short, sturdy legs, turtles have flipper-like limbs that help them to swim more efficiently – though this makes getting around on land far more difficult.
Tortoises live exclusively on dry land and are usually found in hot, dry environments. Their shells are not streamlined like marine turtles but are instead shaped like a dome, and they have column-shaped feet with claws.
Terrapins are the semi-aquatic, freshwater-living versions of turtles. They live in either fresh or slightly salty (brackish) water, and you will find them by waterholes, ponds or lakes. They tend to swim quite often, but also spend their time on land, basking in the sun and occasionally burrowing in the mud.
Reference: What’s the difference?
Notes from some more lookup…
Interestingly, their shell – unlike claws, nails, horns and beaks – contains nerves, so they are quite receptive to feeling through their shells.
Why do they bask in the sun…?
Turtles, like the majority of reptiles, are unable to regulate their own body heat and in order to stay warm, reptiles will bask in this sun. They also need artificial sunlight in the form of UV light in order to synthesize vitamin D3 and so grow healthy bones and shells.