Turtles

Class: Reptilia
Clade: Sauropsida
Order: Testudines(Latin for pot) (Chelonii) 

Turtles are generally regarded as an aquatic reptile with a characteristic shell and varying adaptations to living and moving in water environments.

800px-Chelonia_mydas_is_going_for_the_air

A marine Green Turtle (Chelonia mydas). image: wikipedia

Tortoises tend to be more land dwelling although they may spend some time in aquatic or semi muddy conditions. 

There is some colloquial dispute over the difference between the terms turtles and tortoises as they both belong to the order Testudines.

800px-A._gigantea_Aldabra_Giant_Tortoise

A giant tortoise ( Aldabrachelys gigantea). image: wikipedia


Reference:

http://en.wikipedia.org/wiki/Turtle