The Department of External Services was an intelligence agency of the United States government, operating both inside and outside the country, which featured prominently in much of MacGyver. Peter Thornton was employed by them for years, and on a DXS assignment when he originally met MacGyver in 1979 (in the Season 2 episode Partners. MacGyver himself was then recruited, and worked for them until 1986, at which point he and Thornton both left to go work for the private Phoenix Foundation.
During his employment, MacGyver worked as a field agent on both foreign and domestic cases. His missions were usually covert and non-violent: he was often hired to acquire or destroy an item vital to national security, such as destroying a downed satellite in Afghanistan (To Be A Man) or a missile component in Central Asia (Pilot) before the communists had a chance to study it, or to help American citizens in danger, such as a journalist in Central America (The Gauntlet), a geologist in Basque Spain (Trumbo's World), or a man on the run from the Mafia (The Prodigal).
However, DXS operations are not limited to this. Peter Thornton has alluded to more high-profile missions in his own past, such as supporting Samantha Lorra's family and their rise to power in her Central American nation (Blind Faith), or involvement in the civil war in Yemen (Family Matter). Other parts of the DXS have proven susceptible to corruption, and have been known to take part in illegal and unsanctioned operations, such as their attempts to kidnap foreign heads of state (Early Retirement), or subvert democratic elections (Blind Faith).