Here's my understanding of the special forces in the US:
1) Delta Force: the creme of the US Army. Half-spy, half-soldier. Operates clandestine operations. Recruits from Green Berets + Army Rangers
2) Navy Seals: sea-warrior. Best in water-oriented warfare.
3) Marine Force-Recon: does similar missions to Seals but goes farther inland.
4) Green Berets: non-clandestine premier soldier. Fluent in foreign language. Interfaces with local groups to execute subversive missions against current occupational gov'ts.
5) Army Ranger: premier light-infantry specialists. Used in greater numbers to execute small-scale/efficient combat scenarios. May back up Delta Force/Green Beret units.
6) Marine: the navy's "sea" soldier. Full-fledge operational force for large-scale campaigns, usually around large bodies of water.
7) Airborne: the army's behind the lines soldier. Full-fledge operational force to fight in large campaigns w.o direct line to logistics.