Server Rack Rails
Almost every time you purchase a server to put in a rack, you will need to purchase rack rails in addition in order to complete your installation. Rack rails mount on the rack itself and then attach to the server to hold the server in the rack. There are some different rack types, you may have a two post or a four post. This is why the rack rails are sold separately from the server.
When you purchase the rails, I highly suggest getting quick mount rails that slide. It makes installation a breeze and becomes a near necessity when you start to have many servers in your rack ( when you can't maneuver a screw driver as well anymore ). Choosing the right rails now will save you a lot of headache later.