Turtling describes a highly defensive strategy in which a team "entrenches" in a more or less fixed location, staying close together, taking advantage of AoE bonus effects such as wards, wells, traps and spirits.
Turtling is commonly used in PvE as well as PvP/GvG. The strategy that is geared more towards penalizing mistakes than in taking the initiative. Since turtling is a defensive strategy, it only works if the opponent is actually attacking. This is quite trivial in PvE, where pulling foes is easy, but in PvP more sophisticated baits are required: Good turtling teams often also send out one or two members to goad the opponent into making a bold move. These members try to give the impression of an overextended frontline, but are actually very efficient at retreat. Turtling is commonly used when approaching the opponent's base.
Turtling is being frowned upon as a "cheap" tactic by some players and pejoratively called "camping".
The term "to turtle" goes back to the testudo formation, a defensive formation used in ancient Roman warfare. ("Testudo" is Latin for "tortoise".)