From that picture, it *appears* to be a Russian Tortoise, which is one of the more common species of pet tortoises. A few more pictures from different angles would make that easier to confirm, though. Here is a nice caresheet from the World Chelonian Trust, they also have many articles and caresheets on their site about other species of turtles and tortoises, and a gallery that may help you make a positive ID on which one you have.