Under British colonial rule, Nigeria was divided into two or three regions (the Southern region was divided in 1939) - the North, mostly Muslim and Hausa, and in the South there was the West, mostly Yoruba (Christian and Muslim), and the East (mostly Igbo and Christian now, probably a higher share of animists back then). Historically the South was more educated than the North, but Northerners made up a good share of the military. At independence it was 3 provinces in a federal system, but the number has multiplied since, in part because oil revenues are shared by province so there's an incentive to have your own province.
In Nigeria's history, it's had a couple of short democratic spells (4-6 years each) followed by military coups in 1966 and 1983. There were democratic elections held in 1993, but General Babangida didn't let the winner take office. Babangida was then pressured to hand power to an interim government, but before elections could be held General Abacha took power in November and dissolved all democratic institutions.
Abacha was a bad guy - he was in charge when I lived about 60 km west of Nigeria, in the wonderful town of N'Dali, Benin. Even though I was so close, I was afraid to go into Nigeria - corruption was terrible, lots of violence, etc. Security was so bad in the airport, the US wouldn't let direct flights go to and from Lagos. His government prosecuted and executed environmental activist Ken Saro-Wiwa. When he died in 1998, he was replaced by General Abdulsalami Abubakar who pretty quickly released political prisoners and held elections.
One of the political prisoners he released was (former general) Olusegun Obasanjo, who had been head of state for 3 years (during which he introduced universal primary education) and in 1979 was the first military leader to hold elections and hand over power to a democratically elected government. I remember reading pamphlets arguing for Obasanjo's release when I was in the Peace Corps in Benin.
Obasanjo won the election in 1999 and re-election in 2003 as the candidate for the People's Democratic Party with over 60% of the vote. His vice-president was Atiku Abubakar (not sure if he's related to Abdulsalami), who had been a supporter of Abacha. Abubakar wanted to be president after Obasanjo was term-limited out, but Obasanjo tried to get the constitution changed so he could run for a 3rd term. That didn't work, but Abubakar left the PDP and joined the Action Congress party in 2006. Obasanjo picked Umaru Yar'Adua to be his successor instead, and Yar'Adua, thanks to Obasanjo's support, won the election as the PDP candidate in 2007. When he died in 2010, the vice president, Goodluck Jonathan (gotta like that name, and gotta like his hat) took office.