A turtle or turtling refers in OGame to a passive defensive playing-style, with focus on strong defenses, rather than large fleets. This type of defense is usually made to create a safe-house for fleets and resources or to prevent great loss when all the fleets have been sent, or by players who do not have that much time to play the game fully. That's why this large build of defense allows for a more 'relaxed' playing style, decreasing the possibility of worrying over possible fleet or resource losses. It allows the player to be "offline" for a few days and up to a week if necessary. Any longer will not endanger the turtle planet itself but the turtler's colonies which produce the resources to build the defenses. Generally speaking most turtlers put their planets close to each other, preferably in the same solar system to shorten transport times.
Turtles come in many shapes and sizes, depending mainly on the age of the universe in question. The largest turtles in the oldest universes have up to a million units of defense on one planet, whereas the turtles of the newer universes have merely 2000 units on one planet.
Often, the larger turtles have discovered that building Death Stars behind a huge wall of defensive fodder creates an almost impenetrable wall, behind which you can hold off even the largest fleets in the universe. However, a large volley of IPM from a group of individuals working together can crush even the strongest turtles, but it has to be noted that even when working in a group the amount of time and resources needed to bring down an uber turtler is so staggering that the turtler must have: 1 - having billion of resources and fleet behind his defenses, or 2 - Being very annoying or an irritating player. If one of these criteria does not meet it is generally considered not honorable to IPM a turtler.