In computing, a server is a computer program or a device that provides functionality for other programs or devices, called "clients". This architecture is called the client–server model, and a single overall computation is distributed across multiple processes or devices.
A rack server, also called a rack-mounted server, is a computer dedicated to use as a server and designed to be installed in a framework called a rack. The rack contains multiple mounting slots called bays, each designed to hold a hardware unit secured in place with screws. A rack server has a low-profile enclosure, in contrast to a tower server, which is built into an upright, standalone cabinet.
A blade server is a server chassis housing multiple thin, modular electronic circuit boards, known as server blades. Each blade is a server in its own right, often dedicated to a single application. The blades are literally servers on a card, containing processors, memory, integrated network controllers, an optional Fiber Channel host bus adaptor (HBA) and other input/output (IO) ports.
A tower server is a computer intended for use as a server and built in an upright cabinet that stands alone. The cabinet, called a tower, is similar in size and shape to the cabinet for a tower-style personal computer. This is in contrast to rack server s or blade server s, which are designed to be rack-mounted.