The computer Case is made of metal and holds all of the computer components together in a compact box. It also serves other use's, safty so that electrical conections and components cannot come into contact and electricute you, keeps dust out of you system and also helps to keep the components cool by allowing the fans to suck in air from the front and dispell it from the back.
There are two Types of Cases AT and ATX.
AT/ATX AT is the oldest type of case that is not used any more in modern computers but their still around so you'll still have to know the difference between AT and ATX. The difference is the type of motherboard that it can take. This is coverd in more detail in the motherboard section of this Web Page.