19 Inch Racks are standardizes enclosures or frames, they are specifically designed to house electronic modules (multiple modules). Every single one of these modules has a front panel that is exactly 480mm (19 inches) wide; the size also includes any edges that are used to fasten the module to the rack frame.
The type of equipment that is specifically design to go in these systems are normally referred to as Rack-mount or a rack-mounting systems.
These units are sometimes referred to as 'Relay Racks' this is because they originated as mounting systems for signalling relays on railroads. Although the actually unit and size of 19 inches has remained consistent over time the actually equipment that goes into it has changed dramatically.
The most common areas you will find 19 Inch Electronic Racks in use will be:
Computing
Telecommunication
Entertainment
Audio
When used for audio equipment it is only on a professional basis, you will not walk into someone's home and find an electronic 19 inch rack, these will only be found in industries containing professional amplifiers, interfaces, effects units and also small scale audio mixers.
In a computer sense these are also unlikely to be found in a home, unless you work from home and require a big server. Also usually industrial based these units are used to house computer Server equipment, this allows for dense hardware configurations but all confined to a small space and without mass wires across the floor space.
Western Electronic Racks come in a different size of 23 inches.