A multiplex or mux (called virtual sub-channel in the United States and Canada, and bouquet in France) is a group of TV channels that are mixed (multiplexed) for broadcast over a digital TV channel and separated out again (demultiplexed) by the receiver. There are two different types of multiplexes, which are closely related but not identical.
In the UK a multiplex (usually abbreviated mux) is a band of fixed width containing a number of channels. In the US, the same arrangement is often described as a channel with virtual sub-channels.
Pay TV multiplexes 
Many Pay TV services on cable television offer multiplex packages, in which a single service offers a number of separate channels to its subscribers. The channels may be distinct, be timeshifted, or offer different views of the same event like overviews of a car race, the view from different drivers' cars, or a view of the pits. Some pay TV multiplexes offer picture in picture (PIP) capabilities to follow the various feeds simultaneously on one screen. If the hardware allows, several channels in one multiplex can be viewed simultaneously.
Pay TV may be on a subscription or a pay-per-view basis.
Digital subchannels 
Analog television channels, whether terrestrial, cable, or satellite, are transmitted uncompressed, and always require the same bandwidth, so there is no reason not to transmit each one separately. Digital television stations are transmitted in a compressed format, so that the bandwidth they require varies from instant to instant; it is more efficient to transmit several channels together so that they share the same bandwidth, each channel using the instantaneous bandwidth it needs, with channels that are currently not transmitting giving up their bandwidth to those that are. A group of channels transmitted within a particular bandwidth allocation is known as a multiplex; or the channels may be called subchannels. Sometimes, when analog transmissions are replaced by digital, the fixed bandwidth of one analog station is allocated to a multiplex; the bandwidth of one analog station is sufficient for several compressed channels.
A set top box or Integrated Digital Television is required to tune in, receive, and demultiplex a channel for viewing. A multiplex can contain half a dozen TV channels yet only uses the same space of one analog channel.
Any programs, not necessarily from the same network, can be multiplexed. Programming from a commercial network that would not otherwise be available in the station's broadcast area can be transmitted.
Digital television multiplexes vary in the number of channels that can be transmitted, based on the bandwidth of the multiplex and the broadcast quality specified for each channel. Digital terrestrial offers the least, digital cable and satellite the most bandwidth.
A single multiplex may carry conventional TV channels, radio, teletext, and sometimes hidden channels carrying data.
In the US the standard for over-the-air digital transmissions is ATSC, digital cable is based on the worldwide DVB standard, DVB-C (C for cable) and transmission via satellite are based on the DVB-S (S for satellite) standard, all use multiplexes to deliver various channels to the viewer. Smaller and newer commercial networks, such as The CW and MyNetworkTV, are available in some markets as digital subchannels of other network affiliates rather than as standalone stations. A multiplex can also carry radio and interactive TV content.
The ATSC standard and its early adoption of HDTV is the best quality available of HDTV programming since the cable and satellite providers in the US use heavy compression to fit as many channels into their multiplexes as possible. ATSC and free-to-air satellite TV is free of charge, while digital cable and Direct broadcast satellite TV do not offer any free content. A typical American ATSC multiplex offers 3 to 4 channels, in most cases one of them is broadcast in HDTV (main TV network channel) and the rest of the channels are broadcast in SDTV.
In Europe multiplexes are used on the DVB-T and DVB-T2 (T for Terrestrial) digital terrestrial standards, on DVB-C digital cable and on DVB-S digital satellite. Publicly and privately owned TV networks use multiplexing to broadcast many digital channels over a few multiplexes using the various digital broadcast standards. In Europe a typical DVB-T multiplex offers 4 or more SDTV channels transmitted simultaneously; if some channels transmit only for part of the day (e.g., a children's channel during daytime, a channel with programs for adults in the evening), many channels may share the same multiplex.