From the Overview: "Special Operations Forces (SOF) are those active duty and reserve component forces of the military services designated by the Secretary of Defense (SECDEF) and specifically selected, organized, trained, and equipped to conduct and support special operations. Special operations frequently require unique modes of employment, tactics, techniques, procedures, and equipment. They are often conducted in hostile, politically, and/or diplomatically sensitive environments, and are characterized by one or more of the following: time-sensitivity, clandestine or covert nature, low visibility, work with or through indigenous forces, greater requirements for regional orientation and cultural expertise, and a high degree of risk."
CRS In Focus, IF10545
Congressional Research Service: https://crsreports.congress.gov/