Lagos is the largest city in Nigeria. It 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.
Lagos still serves as the main area of activity because of it's convenient location, and cosmopolitan culture. Most of the business is located on the islands, although the majority of the popluation live on the mainland.
You can fly in from most European cities, and from South Africa and Kenya.
Use busses to move around the city. During the day, they are safe (in terms of crime) and cheap. Taxis cost a lot more, and are safer at night.
For moving around a single area, use Okada. These motorbikes are quick and save a lot of walking. You can safely half their first price, and usually there are lots to choose from. Choose an older driver as the younger ones are cowboys.
Lagos is for tourists like Germany is for humour. It's a sterotype and not really true, but you have to put the effort in to see it.
There are a lot of great beaches. Especially if you make the effort to get outside the side (like Badagari). Inside the city, Lekki is about as good as it gets.
There are also lots of markets to see, although it can be very tiring. Ususally safe during the day.
Nigeria is the only place in the world that Guinness is brewed outside of Ireland. And they do it pretty well, although it's not the same product.
Beer is actually big business in Nigeria, although the move toward evangalism is making its mark. Lagos is relatively unaffected due to it's cosmopolitan nature.
The other cheap drink of choice is Gin, which is locally made. Some locals will swear to it making their step uncle's dog blind, though, so be careful.
Tourism, she's not big, in Lagos. Unless you're a rich American, hotels are really not an option. They're either expensive or not too safe.
The best thing to do is stay with people you know, which is fairly easy since Nigerians are relatively hospitable.
Definitely head for the Africa's Shrine, with Femi Kuti, son of the legend (what's his name again???)
Also worth seeing, are Lagbaja in Ikeja, Oyingbo Road.