What is 1U
Your server(s) will be mounted in the rack. A rack is a metal framework that houses electronic equipment and usually contains bays designed to piece of equipment like a server.
The space used by your server(s) is measured in Rack Units (RU or "U-Space"). This is the unit of measurement used for defining the vertical space used. A 'U' is equivalent to 1.75 inches (4.45cm).
1U Server= 1.75" of Vertical Rack Space
2U = 3.5" of Vertical Rack Space
4U = 7.00" of Vertical Rack Space
A full rack can contains 40 to 42U
1/4 Rack=10U (10 servers of 1U)
Half rack =20 U (20 Servers of 1U)
So, if you have to buy your own server you have to choose a rackmount server (or rack mounted server) it has been designed to live on a rack.