Servers 101. Can someone give me a quick run through?

I have these old xp and win7 machines. I'm planning to make home servers. What can I use it for?

I want to make them because I wanted to broaden my IT knowledge. I'm a software engineering student but I like IT more. I want to know how to maintain, create and setup servers. How to protect them also. Thank you for your reply!

EDIT I MANAGED TO BUILD THIS OUT OF THE SCRAP: MSI H61M-P20 (G3), Intel i3 3220, 8GB DDR3