The Northern Alliance (who prefer the title United Front) are a group of factions with the common goal of toppling the Taliban regime currently in power in Afghanistan. They have been fighting against Taliban control for many years, ever since the Soviet Union withdrew from the country. In November of 2001, they were successful in advancing to control much of the northern part of the country, including the capital Kabul. This was facilitated by extensive bombing of Taliban forces and military infrastructure by the United States during the 2001 U.S. Attack on Afghanistan.
Many are ethnic Tajiks (which may be a factor in Tajikistan's willingness to help the United States in the conflict), Uzbeks and Hazaras - ethnic minorities in Afghanistan. Some factions are from the former mujahedin, others from different minority groups, and other forces (including remnants of the Soviet-backed Communists?).
The Northern Alliance enjoys support from India, while the Taliban were close to Pakistan until the September 11, 2001 Terrorist Attack when Pakistan backed off due to pressure and the hope of debt relief.