Server architecture
This wiki is mainly focused on the viewer, since that is the portion of the system that is open source. However, because the viewer interacts with the Second Life servers in somewhat complicated ways, it is helpful to have an understanding of what components do what on the server side.
Much of what you'll need to understand is actually covered in the protocol documentation. In particular, take a look at Authentication Flow, which outlines the process by which the viewer establishes connections with a number of components.