Turtles are an order of reptiles characterized by their bony shells. A tortoise is a turtle that lives on land. There are some complicated technical issues surrounding these terms, and the usage habits vary among British, American, and Australian English. But as long as you’re not a biologist, you’re safe referring to any turtle that lives primarily on land as a tortoise.
There are also sea turtles—obviously, turtles that live in the sea—and terrapins, a species of turtle native to coastal swamps in the southern U.S.
Of course, in American English, we tend to use turtle to describe all turtles, even those that could be described as tortoises.
Examples
Sea turtles, among other wildlife, will eat shriveled or exploded rubber balloons; they look like jellyfish. [Charleston Post Courier]
At issue, as outlined in the organization’s press release, are concerns over loss of the habitat of the threatened desert tortoise. [Yahoo! News]
In mid-August the Observer newspaper reported that leatherback turtles were swimming across the Atlantic from their nesting grounds in the Caribbean. [Victoria Times Colonist]
So my father must have figured he’d gotten a break when, one day, while in the car returning from a swimming lesson, my brothers and I spotted a tortoise crawling along the sidewalk. [Christian Science Monitor]