From The TV IV
United Nations Intelligence Taskforce (aka UNIT) is a military organization based in England where The Doctor finds most of his higher government allies. UNIT's function is to explore alien and unusual threats to Earth. First referenced during one of The Second Doctor's serials, UNIT became very important when The Doctor was forced to stay on Earth, the first three seasons of The Third Doctor's reign. For all of the Third and Fourth Doctor's tenure UNIT was overseen by Brigadier Lethbridge Stewart. In the episode Mawdryn Undead during The Fifth Doctor's reign, it was revealed he retired (which sent the Who Fandom into a huge firestorm as the dates given in that episode regarding UNIT run counter to those during the third Doctor's). In the episode Battlefield Brigadier Winifred Bambera was in charge.
In the new series, UNIT has made a few small cameos. First as the government website The Ninth Doctor has Mickey hack into during World War Three and then where Prime Minister Harriet Jones goes after the Sycorax threaten to enslave humanity in The Christmas Invasion