In A Nutshell
Turtles and tortoises are both reptiles in the order Testudines that are characterized by the bony or cartilaginous shells formed from their ribs. Differentiating between the two can become complicated due to regional differences in semantics, but there are some easy ways to separate the groups scientifically. All tortoises are turtles, but not vice versa. The word “tortoise” specifically refers to any species within the family Testudinidae, where individuals are typically herbivorous and terrestrial, with blunt feet equipped for travel on land.
The Whole Bushel
All turtles belong to the order Testudines, which is also sometimes known as Chelonii. All tortoises are turtles because they also belong to the order Testudines, but the word “tortoise” refers more specifically to the family Testudinidae. The word “terrapin” is sometimes used in common names to refer to turtles that seem to fall between tortoises and other turtles characteristically.
Typically, turtles spend their lives in or around freshwater, marshlands, or the ocean. Because of this, a turtle’s feet will usually have webbed toes if they are not one of the species that has fins. Turtles typically mate and lay their eggs in or around water. Though it varies, most turtles are primarily omnivores, and their diet can consist of plants, insects, and fish.
Tortoises, on the other hand, are strictly terrestrial. They cannot swim, and deep water could even cause them to drown. A tortoise would only wade into water periodically to clean itself or to drink. They have blunt, club-shaped feet that are well equipped for traveling across land. They also have claws which they use to dig burrows to use for sleeping or shelter from bad weather conditions. Unlike turtles, tortoises are primarily herbivorous (though they have been known to eat insects, worms, or carrion occasionally).
Regional differences in semantics are what cause the most confusion when differentiating between turtles, tortoises, and terrapins. They are referred to differently in American English and British English, though modern communication has begun to blur the lines between the two.
In the US, a turtle is found in or around water while a tortoise is found on dry land. A terrapin is a turtle found in brackish water. Generally, you can tell them apart by looking at the back legs of the reptile. If they are webbed, call it a turtle. If they are stumpy and elephant-like, call it a tortoise (though the USA only has one native, the gopher tortoise). In the UK, the term “terrapin” is used to describe all chelonians that live in freshwater environments. The term “turtle” refers only to ocean dwellers, and “tortoise” refers to terrestrial species.
Australia is close to the UK in this sense. Australians usually use the term “tortoise” to refer to everything that isn’t a sea turtle, though there aren’t any land chelonians native to the continent. Turtles, tortoises, and terrapins all fall under the order referred to as turtles, but you can use some of the qualities we’ve talked about for easier identification. If it basks, swims, has webbed feet or fins, and is primarily omnivorous then it is a turtle. If it is strictly terrestrial, has blunt non-webbed feet, and is primarily herbivorous then it is a tortoise.