Pashtun: The Pashtun are where the Taliban draw their support from because the Taliban leadership is from the Kandahar-area in southern Afghanistan. Pashtun form around 40% of the population (last census was in 1979 so who really knows) and have traditionally run Afghanistan. Found in the south and east. Pashtun are also found in Pakistan and the border drawn in the nineteenth century does not mean much to the tribes along it.
Tajiks: Tajiks are primarily centered in the upper east but spread throughout to the west and make up about a forth of the population. Tajiks have gained credit in Afghanistan for their fierce resistance to the Soviet invasion and Taliban rule. Ahmad Shah Massoud embodied this movement and was also opposed to Osama bin Laden (Massoud was killed two days before 9/11 because al Qaeda knew how important Massoud was). Tajiks consider themselves the most educated of all Afghans. In the west there is a rivalry between Pashtuns and Tajiks for political control.
Hazara: Hazara are the only majority Shia (Shiites) in Afghanistan. They are found mostly in the center of Afghanistan and form about twenty percent of the population. They have long ties to Iran because of the shared faith. Almost a million Hazara are in Iran. Taliban have an intense hate of Hazara because during the late 1990s the Hazara managed to massacre the Taliban in several battles.
Uzbeks: Uzebeks are found in the north and make up ten percent of Afghanistan. Many Uzbeks supported the Soviet-backed government. Abdul Rashid Dostum, backed by most Afghan Uzbeks, have fought with pretty much every side in Afghanistan since 1979. Uzbeks are the only ethnic group that "looks north" and has copied many traits from their brethren in Uzbekistan (more Central Asian than Southern Asian).
Baluchis: Baluchis live in the deserts of the southwest. Hard to pin down their numbers because of semi-nodamic lifestyle. Some Baluchis serve as smugglers for the Taliban. Have ties with Baluchis in Pakistan and Iran. Baluchis are currently fighting low level wars against Pakistan and Iran for greater independence. They are heavily involved in the drug trade.