Turtling is a gameplay style meaning, going for resources early, building alot of defences which nearly cover all threats and having big bases. A turling player mostly stays limited with a few mass extractors. He build it tight and paced together, with shield generators, anti-nukes, point defences, Anti-air turrets, mobile units, Interceptors/interceptor/bomber, gunship, land units, artillery.In the older Supreme commander games the difference could be seen that a turtling player would go for resources by teching up mass extractors earlier then needed. A more detailed guide can be found at www.supcom.wikia.com under the strategy section
I still believe in this game that the UEF can turtle best, because their short range artillery, it fires quick and it can't be ignored.
An extreme version of turtling is to forgo building mobile military units entirely and instead use all one's resources on an impenetrable defense line and large quantities of artillery and other indirect fire structures. One advantage of this strategy is that artillery does not require resources to fire. This means that even if the player's resource generation and unit production are disabled, they could still fight and possibility win.
The choice of faction has a large impact on the feasibility of this strategy. The UEF has the fortified artillery station which, when used in large numbers, can be a very effective defense on it's own and can also help soften incoming armies for point defenses. However, if playing as UEF one will need to send engineers out to build sacrificial radar stations in order to target far away bases, this can obviously be challenging with no military units to guard the engineer.
Sacrificial radar stations are not needed when playing as the Cybrans if the player researches shell camera. However, the Cybrans do not have a medium range artillery to soften incoming armies, meaning their point defenses must be able to take waves of experimentals without breaking.
This strategy does not work nearly as well, if at all, for Illuminate players as that faction does not have long range artillery. An illuminate player would have to rely on nukes and building tactical missile launchers close to the enemy base.