What's the difference between a turtle and a tortoise?

We visited a local initiative in Nungwi to save Zanzibar's turtles. Fishermen who ended up with turtles in their nets by accident, send them here to recuperate before they a released into the wild again.

It’s basically a large saltwater lagoon, hemmed in by coral and refreshed with water every high tide. There were 15 Green Turtles swimming around, some of them quite big – the oldest was 25 years old. 

Younger turtles are protected in two small, covered ponds – I was able to hold one of the smaller ones, but it was already very heavy, like an iron brick! 

The real babies – just 20 days old – were zipping around in another pool, their flippers far bigger than their tiny shells.

P.S. Turtles live primarily in the water, whereas tortoises are land dwellers.