Do you think you know what a turtle is?
A dictionary definition sounds like this: “any of various chelonian reptiles, especially those of the marine family Chelonidae, having a flattened shell enclosing the body and flipper-like limbs adapted for swimming.” That's for the “English turtle.” US and Canadian turtles are “any of the chelonian reptiles, including the turtoises and terrapins.” 
There is a better understandable (and funnier) illustration of this subject in Richard Dawkins' book with the title The Greatest Show on Earth . Quoting George Bernhard Shaw's saying that “England and America are two countries divided by a common language,” Richard Dawkins continues:
In Britain, turtles live in the sea, tortoises live on land and terrapins live in fresh or brackish water. In America all these animals are ‘turtles,’ whether they live on land or in water. ‘Land turtles’ sounds odd to me, but not to an American, for whom tortoises are the subset of turtles that live on land. Some Americans use ‘tortoise’ in a strict taxonomic sense to refer to the Testudinidae, which is the scientific name for modern land tortoises.In Britain, we'd be inclined to call any land-dwelling chelonian a tortoise, whether it is a member of the Testudinidae or not.
What a mess! In case you wonder, Australians use the word turtle in yet different ways.
Any solution to this linguistic jumble? Zoologists, in their research, use the term chelonian. Broad-based aspects of the conservation and biology of these animals are covered in an international scientific peer-reviewed journal: Chelonian Conservation and Biology .
Like the scientific language, the German language has one word for all: Schildkröte for turtle, tortoise and terrapin . Schildkröte literally means shielded toad. A terrapin is a Sumpfschildkröte, Sumpf meaning swamp or bog.
Keywords: languages, terminology, biology, Testudinidae, Chelonidae, nomenclature, taxonomy, classification, confusion.
References and more to explore
 Dictionary.com: turtle [dictionary.reference.com/browse/turtle?s=t].
 Richard Dawkins: The Greatest Show on Earth. Free Press, New York, 2009.
 Chelonian Conservation and Biology [www.chelonian.org/ccb/].
 Edmund Launert: Biologisches Wörterbuch. Verlag Eugen Ulmer, Stuttgart, 1998.