A 19-inch rack is a standardized frame or enclosure for mounting multiple equipment modules. Each module has a front panel that is 19 inches (482.6 mm) wide, including edges or ears that protrude on each side which allow the module to be fastened to the rack frame with screws.
1U equals 1.75-inches (44.45mm) of rack height. Therefore, a 2U rack mount height would be 2 x 1.75", which equals 3.5-inches high. A 3U height would be 3 x 1.75" = 5.25-inches.
Rack unit is a measure used to describe the height of a server, network switch or other similar device mounted in a rack. One rack unit is 44.45 mm (1.75 in). One rack unit is commonly designated as "1U"; similarly, 2 rack units are "2U" and so on. The size of a piece of rack-mounted equipment is usually described as a number in "U". The number of rack units available in 8-inch space is 8/1.75 or 4U (fractional part has been trimmed).
Server rack is useful for arranging the servers in a structured manner, one above the other, saving precious floor space.
A Blade server is self-contained computer server, designed for high density, and efficient space utilization. A blade enclosure provides services such as power, cooling, networking, various interconnects and management. Together these form the blade system.
A tower computer will have diagnostic lamps (LEDs) at the rear of the tower. Other features of tower computer include ability to add additional components (such as HDD, or DVD drive), better internal cooling due to more space, and more power because bigger power supply may be used to accommodate future expansion.