AV receivers or audio-video receivers are one of the many consumer electronics components typically found within a home theater system. Their primary purpose is to amplify sound from a multitude of possible audio sources as well as route video signals to the user's TV from various sources. The user may program and configure a unit to take inputs from devices such as DVD players, VCRs etc. and easily select for which source he or she wants to route to their TV and have sound output.
The term receiver basically refers to an amplifier that is capable of receiving an audio & video signal from various devices. The basic functionality is to receive an audio signal and amplify the same and to allow pass through of the video signal onto a display device such as a projector or a television.
As home entertainment options expanded, so did the role of the receiver. The ability to handle a variety of digital audio signals was added. More amplifiers were added for surround sound playback. Video switching was added to simplify switching between devices. Within the last few years, video processing has been added to many receivers.
The term audio/video receiver (AVR) or Home Theater Receiver is also used to distinguish the simpler stereo receiver from the multi-channel audio/video receiver (home theater receiver), though the primary function of both is amplification.
The AV receiver is classified as an audio frequency electronic amplifier. But with the rapid addition of several features, AV receivers now generally have significant additional functionality.
Stereo receivers have two channels of amplification, while AV receivers may have more than two. The standard for AV receivers is five channels of amplification. These are usually referred to as 5.1 receivers. This provides for a left, right, center, left surround and right surround speaker to be powered by the receiver. 7.1 receivers are becoming more common and provide for two additional surround channels, left rear surround and right rear surround. The ".1" refers to the LFE (low-frequency effects/bass) channel the signal of which is usually sent to an amplified subwoofer unit. 5.1 and 7.1 receivers don't usually provide amplification for this channel. Instead, they provide a line level output. There are various standards for rating the output power of receivers. Different countries have different rules on how manufacturers specify the output ratings. It is not always possible to use these ratings to compare two products. Due to a number of factors such as real world behavior of speakers and dynamic headroom it is possible for an amplifier with a lower rated power to play more loudly than one with a higher rated power. Differences in output power are not always as significant as they may look. It takes ten times the output power for the sound to be perceived as twice as loud. If 1 watt of output yields a sound pressure level of 90 dB, it takes 10 watts to get an SPL of 100 dB and 100 watts to get an SPL of 110 dB. A 110 watt amplifier will not play 10% louder than a 100 watt amplifier. Most receivers use class AB amplifiers. Some manufacturers are now producing receivers using class D amplifiers. Class D amplifiers are more efficient and can be made smaller and lighter than an equivalent class AB amplifier. There are also other designs such as class G and class H. Class G and H are variations on the conventional class AB design. Class G has two sets of power supply rails. Normally the power amp is fed from the lower voltage supply. This helps keep power dissipation in the output transistors down. When the signal exceeds the lower supply voltage, the amp switches to the higher voltage supply so the signal can be reproduced without clipping. With a class H design, the supply rails are variable rather than two discrete steps. The signal actually modulates the supply voltage.
AV receivers usually provide one or more decoders for sources with more than two channels of audio information. This is most common with movie soundtracks, which use one of a variety of different types of encoding formats.
The first common soundtrack format was Dolby Pro Logic, a surround sound processing technology. This format contains a center channel and a surround channel mixed into the left and right channels using a process called matrixing, providing a total of four channels. Receivers with Dolby Pro Logic decoders can separate out the center and surround channels from the left and right channels.
With the introduction of the DVD, the Dolby Digital format became a standard. Dolby Digital ready receivers included inputs and amplifiers for the additional channels. Most current AV receivers provide a Dolby Digital decoder and at least one digital S/PDIF input which can be connected to a source which provides a Dolby Digital output.
A somewhat less common surround sound decoder called DTS is standard on current AV receivers.
When Dolby Labs and DTS introduced technologies to add a rear center surround channel, these technologies found their way into AV Receivers. Receivers with six amplifiers (known as 6.1 receivers) will typically have both Dolby and DTS's technologies. These are Dolby Digital EX and DTS ES.
Dolby introduced Dolby Pro Logic II to allow stereo sources to play back as if they were encoded in surround sound. DTS introduced a similar technology, NEO:6. These decoders have become common on most current receivers.
As the number o f playback channels were increased on receivers, other decoders have been added to some receivers. For example, Dolby Labs created Dolby Pro Logic IIx to take advantage of receivers with more than five channels of playback.
With the introduction of high definition players (e.g. Blu-ray Disc and HD DVD), yet more decoders have been added to some receivers. Dolby TrueHD and DTS-HD Master Audio decoders are available on many receivers.
DSP effects 
Most receivers offer specialized Digital Signal Processors (DSP) made for handling various presets and audio effects. Some may offer simple equalizers and balance adjustments to complex DSP audio field simulations such as "Hall", "Arena", "Opera", etc. that simulate or attempt to replicate as if the audio were being played in the places through use of surround sound and echo effects.
AV inputs/outputs 
There are a variety of possible connections on an AV receiver. Standard connectors include:
- Analog audio (RCA connector, or occasionally XLR connector )
- Digital audio (S/PDIF; TOSLINK or RCA terminated coaxial cable)
- Composite video (RCA connector)
- SCART video (primarily used in Europe and very uncommon in many other parts of the world)
- Component video
- USB (usually involves special computer circuitry to read video formats from a file system)
Analog audio connections usually use RCA plugs in stereo pairs. Inputs and outputs are both common. Outputs are provided mainly for cassette tape decks.
Analog audio connections using XLR(Balanced) connectors are uncommon, and usually found on more expensive receivers.
Composite video connections use a single RCA plug on each end. Composite video is standard on all AV receivers allowing for the switching of video devices such as VHS players, cable boxes, and game consoles. DVD players may be connected via composite video connectors although a higher bandwidth connection is recommended.
S-Video connections offer better quality than composite video. It uses a DIN jack.
SCART connections generally offer the best quality video at standard-definition, due to the use of pure RGB signalling (although composite and S-Video may alternatively be offered over a SCART connector). SCART provides video and audio in one connection.
Component video has become the best connection for analog video as higher definitions such as 720p have become common. The YPbPr signalling provides a good compromise between resolution and colour definition.
HDMI is becoming common on AV receivers. It provides for the transmission of both audio and video. HDMI is relatively new technology and there are reported issues with devices not properly working with each other (referred to as hand-shake issues between devices), especially cable/satellite boxes connected to a display through an AV receiver. Different levels of support are provided by receivers with HDMI connections. Some will only switch video and not provide for audio processing. Some will not handle multi-channel LPCM. Multi-channel LPCM is a common way for Blu-ray Disc and HD DVD players to transmit the best possible audio.
Video conversion and upscaling 
Some AV Receivers can convert from one video format to another. This is commonly called upconversion or transcoding. A smaller number of receivers provide for de-interlacing of video signals. For example, a receiver with upconversion, deinterlacing and upscaling can take an interlaced composite signal at 480i (480 lines per frame sent as a field of 240 even numbered lines 0,2,4,8...478 followed by a field of 240 odd numbered lines 1,3,5,...479) and convert it to component video while also deinterlacing and upscaling it to a higher resolution such as 720p (720 lines per frame with all lines in normal sequence 0,1,2...719).
Radio on AVRs 
AV receivers though primarily used for amplification may or may not have an inbuilt AM/FM radio tuner among other features such as LAN connectivity for various Internet applications and some with multi-room audio solutions.
Even though some AVRs may have an AM/FM tuner it is not a primary or mandatory function as an AVR still remains a amplifier.
Some models have HD Radio tuners.
Some models have Internet radio and PC streaming access capabilities with an Ethernet port.
Some popular brands of AV receivers are (not in any particular order) Pioneer, Sony, BOSE, Bang & Olufsen, Yamaha, Denon, Onkyo, Harman Kardon, Klipsch, NAD, Marantz, Sherwood, Arcam, Cambridge Audio (UK), Rotel, Anthem, Outlaw, McIntosh, etc. This list does not cover every brand present in this category.