Turtles are the most popular reptilian pets. More turtles are kept as pets than snakes and lizards combined.
There are three terms commonly used to categorize the four footed shelled animals that belong to the taxonomic order Chelonia. They are turtles, tortoises and terrapins, but which species falls into which category depends largely on where you are talking about them. For instance in the US the term terrapin is rarely used only applying to one species the Diamond Back Terrapin (Malaclemmys palustris) but in Great Britain it is used to describe chelonians that are found in fresh and brackish water habitats.
The information that follows will refer to chelonians that live a totally land based existence as tortoises and those that spend at least some of their time in fresh or brackish water as turtles. Besides the need for access to water the diet of these two groups is also different tortoises are vegetarians and turtles are omnivores meaning they eat both meat and plant material. The percentage of each depends on the species and even the age of the turtle.
Unlike other reptiles that may normally spend days or weeks between meals the chelonians need to feed daily. They also like some lizards need sunlight to synthesize vitamin D so any that are kept indoors require a special full spectrum light that mimics the sunlight they would get in the wild.
The chelonians need a bigger enclosure relative to their size than the other reptiles and unlike many other reptiles are not stressed by an enclosure that is too large. The usually recommended minimum size is at least 5 times as long and 3 times as wide as the length of the turtle or tortoise. While they aren't known for acrobatics they have strong legs that allow them especially smaller ones to hoist themselves over an obstacle so the enclosure should be at least twice their length higher than the top of any solid climbing surface and one times their length above any water.
Chelonians are cold blooded like the rest of the reptiles but tend to be much less sensitive to temperature and humidity. Their comfort zone falls somewhere between the low 70's to mid 80's and many will stop eating if it gets warmer. Of course the arid desert species prefer temperatures closer to the upper end of the zone and aquatic and forest species tend to do best lower than this. Humidity is usually the opposite of temperature with the arid species preferring lower levels and the others higher.
There is only one species of freshwater turtle built exclusively for the water. It is the Pig-nosed Turtle (Carettochelys insculpta) that is found in Northern Australia, Southern New Guinea and Irian Jaya inhabiting remote forested waterways that typically have little or no bank. They have adapted by developing flippers.
We've categorized the turtles below as either aquatic or semi-aquatic. The aquatic turtles will spend most of their time in the water are strong swimmers usually only coming out to bask on floating logs or the shoreline. The semi-aquatics will spend most of their time on land but occasionally go into the water mainly to cool off or relieve themselves. They aren't nearly as good at swimming and prefer to occasionally soak instead. The true tortoises are listed separately due to their different care and dietary requirements.