Lagos is a massive city of somewhere between 10 and 20 Million people. Really, nobody can tell you accurately because the last census was in the 70's, although census volunteers have been seen walking the streets in late 2004.
Originally, the city was rules by kings, which were conquered by neighbouring Benin. After this, the Portuguese and English colonies successively made puppets of these locally revered leaders, to further their trading needs.
Lagos was famous for being the trading hub of many spices and metals. For a long period, it functioned as a slave trading post too, although it's hard to find remnants of this history in contemporary Lagos.
Currently, Lagos is the major business centre in Nigeria, although it is no longer the capital city. This is because the new 1994 civilian government found corruption endemic and decided to start afresh in Abuja. 10 years later, it looks like Abuja is going the same way.