A rack, in an IT (information technology) context, is a supporting framework that holds hardware modules. In this context, racks typically contain servers, hard disk drives and other computing equipment. Racks make it possible to contain a lot of equipment in a small physical footprint without requiring shelving.
The standard size for racks is nineteen inches across, although the ETSI (European Telecommunications Standards Institute) standard is 23”. Nineteen inch racks are typically 73.5 inches high. For this purpose, the size of equipment designed to be rack-mounted is specified in terms of rack units (U), which equal 1.75 inches. Racks contain multiple mounting slots called bays, each designed to hold a hardware unit secured in place with screws. (Definition continues below the image.)
Nineteen-inch racks are used in data centers and server rooms as well as ISP, telecommunication and computing facilities. Other uses for racks include audio and video equipment such as amplifiers and small scale audio mixers and industrial power, control and automation hardware.
The main alternatives to rack-mounted servers are tower servers and blade servers. Hyper-converged infrastructure, a newer technology, involves a software-centric architecture that is tightly integrated with compute, storage, networking and virtualization resources within a single physical device.
See an introduction to server racks: