UNIT (United Nations Intelligence Taskforce) was a military organization, formed around 1970 under the auspices of the United Nations to combat the growing threat of alien invasions. The necessity of such an organization was demonstrated by the then-recent takeover attempts by the Great Intelligence and the Cybermen in London.
UNIT was originally under the command of Brigadier Alistair Lethbridge-Stewart, with Sgt. Benton and Captain Mike Yates as his subordinates. The brigadier led UNIT in its busiest days, the 1970s, when numerous alien and monstrous threats arose, including the Nestene Consciousness, the Silurians and the Sea Devils, the Master, the Daleks, and many more. The Third Doctor was an aid during these times, along with his assistants (Liz Shaw, then Jo Grant, then Sarah Jane Smith).
However, shortly after the Third Doctor regenerated into his next incarnation, he left UNIT. Despite that, he kept ties with them, and subsequent Doctors assisted UNIT on other occasions. Lethbridge-Stewart retired in 1976, passing UNIT leadership to Colonel Crichton, who in turn retired by 1997 and was replaced by Brigadier Bambera.
At some point prior to the 2009 Sontaran invasion, UNIT changed its name from United Nations Intelligence Taskforce to Unified Intelligence Taskforce, although the UN still contributed funding to the organization