In a home theater the amplifier is equivalent to the brain of the system. Depending on the type of amplifier you purchase they all provide different types of services. Generally an amplifier takes a small amount of energy and converts it to a larger amount of energy. Simply put an amplifier takes audio input and pushes the audio through the amplifier into the connected speakers.
A special type of amplifier typically used in home theater/media room environments is called an AV (Audio Video) receiver. This type of receiver allows input of multiple sources such as Satelite/Cable, DVD (Blue-ray, HD-DVD), Tivo into one device. Each device is then routed to the source video and audio devices through the remote.
Typical AV receivers have at least 5 audio video inputs and 1 video output along with speaker terminals. Depending on the features installed different types of inputs are supported for each grade of receiver. Starter receivers will generally have analog RCA audio and RCA or S-VIDEO video input terminals. Mid range to high grade receivers will have have optical/coax digital audio inputs and component and/or HDMI (Video and Audio) for video.
Modern receivers allow you to use the lower performance inputs and output (bridge) through an higher quality output. For instance if you input a video source through RCA the output will be bridged through component monitor out connector to your television monitor. While it can lead you to believe that your content will look better because its being routed through the component output this is not the case. The rule of thumb is what you put in as far as quality wise is what you will get out. The receiver may clean up the picture a little bit not it can only do so much.
AV Receiver Amp
While normal amplifiers are either in mono or stereo a typical home theater amplifier will have at least 5 amps. Each amp controls a single channel on the receiver. A channel can be defined as Left, Center, Right, Left Surround and Right surround. Cheaper amplifiers do not use independent amps per channel but higher grade amps use a independent amp per channel providing clearer and more powerful sound reproduction. When searching for the right AV receiver always check the fine print to see if your choice has independent amps per channel.
AV receivers usually provide one or more decoders for multichannel surround playback. Most receivers have the ability to decode Dolby Prologic, Dolby Digital or the less commonly known DTS. Witht he popularity of the DVD format Dolby Digital has become the standard digital audio in home theaters. Most receivers will have a S/PDIF electrical coax cable or TOSLINK (optical fiber) to transmit digital audio to be decoded by the receiver.