United States - ARMY
De Opresso Liber - Liberate The Oppressed
Today's US Army Special Forces are also known as Green Berets, though this is not an official name for them. USSF, as the acronym summarizes, is a branch of the US Special Operations Command, known as USSOCOM. They trace their lineage through many years of specialized warfare, all the way back to a group known as the Mosby's Raiders, the same group the Army Rangers also claim as their predecessor. This group conducted deep Unconventional Warfare during the civil war. A more recent example of this legacy is the Office of Strategic Services, or the OSS, during World War 2. This group performed a myriad of tasks, one of which was guerilla warfare against the Nazi party in Europe. It was not until Vietnam that Special Forces became a household word, however. As idolized in the movie by John Wayne, The Green Berets, and as immortalized in numerous books and songs, these men performed dangerous and often secret operations in and around the country. The importance of the missions, as well as the undeniable courage of the men involved is exemplified by the impressive amount of Congressional Medal of Honors awarded to Special Forces soldiers during this conflict. Today, the US Army Special Forces continue to perform these and other missions around the world in every conflict and many conflicts undeclared.
The United States Special Forces have 5 primary missions, 4 secondary:
UNCONVENTIONAL WARFARE: These missions are typically performed deep behind enemy lines, typically for long durations. They include but are not limited to:
escape and evasion
espionage and sabotage
DIRECT ACTION: Any action directed at a designated target or unit, typically for short durations. These can be:
direct combat operations
raids, ambushes, target seizure, and direct combat
SPECIAL RECONNAISSANCE: Covert operations designed to avoid contact and spearhead operations by assisting headquarters in planning combat operations.
FOREIGN INTERNAL DEFENSE: Assisting and instructing allied foreign forces. Teams usually live and travel with allied units in foreign territory.
COUNTER TERRORISM: Deterring and exposing terrorist operations.
The Special Forces crest is two crossed arrows and a 2-sided knife in the middle. These items represent the Special Forces soldier's primary role of unconventional warfare, that is, operations other than the standard unit operations. The branch insignia is two crossed golden arrows.
The core of any Special Forces unit is the Special Operations Detachment Alpha - A Team. It is a team of 12 soldiers specializing in different jobs.
2 Weapons Sergeants - 18B
2 Engineering Sergeants - 18C
2 Communication Sergeants - 18E
2 Medical Sergeants - 18D
1 Operations and Intelligence Sergeant - 18F
1 Operations Master Sergeant - 18Z
1 Executive Warrant Officer - 180A
1 Commanding Captain - 18A
The MOS (Military Occupation Specialty) series designation is the 18-series.
Other Units Classified as Special Operations Forces:
Airborne - Some Units
LRSD - Long Range Recon Special Detachment(I believe)
Tactical Combat Controller - Air Force
Special Fores Operational Detachment Delta a.k.a. Delta Force
Note: Delta Force also falls under the "cover name" CAG, or Combat Applications Group.
For a wonderful w/u and comprehensive summary of Special Forces see Green Berets.