Im having trouble understanding a tutorial, how does sockaddr struct work

[devonrevenge]

In part 12 of
http://net.pku.edu.cn/~course/cs501/2011/code/BSD_Socket.t/sockets.pdf

it doesn't quite explain how one builds that struct and the struct within it, I just want to set up each stage and understand it, its quite hard to find the tutorials that walk you through every detail carefully, any help is most welcome.

I doubt i will be entering this competition then :/