they are Red Ear turtles or sliders as they are called in some places.
They are everywhere in the fresh water bayous. They can get large. They make stinky pets.
As these are the same ones that you can buy in the pet stores for a few bucks indeed.
It is illegal for pet stores to sell a Red-eared Slider that has a carapace (shell) less than 4 inches in diameter. This is because of the risk of salmonellosis. So they arent the best choise for pets for children.
Ive seen these down here in the lakes grow up to about 18 to 20 inches across. They can destroy the fish in a small pond in a very short time. They can live to be 50 - 70 years old.