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. 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.
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.