4-Post Cabinet Racks
Also known as "server racks" or "equipment racks" are enclosed cabinets mainly designed for housing servers, UPSs and other heavy equipment that requires 4-point mounting, ventilation, and physical protection.
4-Post Open Frame Racks
Basically, the same as cabinet racks, except as the name emplies they are "open" and not enclosed.
2-Post Relay Rack
Also known as "open bay racks", are typically used for lighter equipment such as switches, patch cabling, and interface terminals.
These come in 2 or 4 post variations with enclosed or open frames, and as the name suggests mount on the wall as opposed to the floor. These tend to be used primarily for Intermediate Distribution Frames (i.e. switches, and patch cabling) outside of the main room/office. They can however be used to mount small servers and other small equipment as well.