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.