"Turtle" is a term used to describe players who spend most or all of the game defending, as opposed to attacking. Generally, Terran is the race that "turtles" the most given the static nature of many of their units (i.e. Siege Tanks, Turrets, Spider Mines). However, turtling is not exclusive to Terran. Zerg can turtle, usually using Spore/Sunken Colonies, Lurkers, and Defilers with Dark Swarm. Protoss can utilize Cannons, Reavers, and Templar for turtling purposes, among other things. Some players revolve their entire game around turtling and playing passively, but in general it is one of the tactics people use to catch up to an opponent who is ahead in army size.
The problem with turtling is that by the time a reasonable amount of defenses are set up, your opponent has had enough time to tech up to units that can overcome those defensive structures. The Zerg guardian surpasses the range of a photon cannon, spore/sunken colony, and missle turret. The Protoss Reaver surpasses the range of a photon cannon, spore/sunken colony, and missle turret. And lastly, the Terran Siege Tank in Siege mode range of a photon cannon, spore/sunken colony, and missle turret.