Rio Grande Turtle & Tortoise Club
Technically speaking, a turtle is aquatic; i.e. lives in or spends most of its time in water, while a tortoise lives on land. However, most Americans use the term "turtle" to describe both turtles and tortoises, and we embrace this common usage here since it is a lot easier and less awkward than always saying "turtle or tortoise." "Terrapin" is another term used to describe either brackish water turtles, e.g., diamondback terrapins, or, in Britain and elsewhere, semi-aquatic freshwater turtles (as opposed to sea turtles) such as red-eared sliders. Sometimes we'll also use the term "chelonian", which refers to the scientific order of reptiles that includes turtles, tortoises, and terrapins.