A relay rack is an "open" style rack that mostly consists of two upright beams with screw holes for mounting equipment (traditionally, relays...thus the name.) Relay racks can be used to mount any rack-mountable devices but you have to remember that A) there is no rear rail, or any way to install a rear rail to support the back of equipment and B) the racks are unstable if the equipment has too much weight too far from the rails.
