a tortoise, terrapin and a turtle are all members of the order Testudines; in many parts of the world the term turtle is used for all three, possibly because testudines is a bit of an academic mouthful - not many of us would call a horse/donkey/zebra an equid either, although those three are distinctive enough in appearance not to get mixed up...

tortoises are not aquatic; the other two are, which is the big distinction..

@TC - they say 'even a tortoise gets nowhere if he does not stick his neck out' - cute; and boy is spot on with his comment...