Multichannel refers to audio streams with more than two channels. The most usual multichannel setups are 5.1 (five main channels: front center, front right, front left, rear right, rear left and one Low Frequency Extension channel) and 6.1 (the five channels from 5.1 + back center).
Formerly limited to movie theaters, multichannel audio has became popular since the introduction of the DVD video format. The two competitors to the title of CD successor, DVD-Audio and SACD, can also offer multichannel content.
There's no standard channel mapping for multichannel streams, and each vendor (Microsoft, Dolby, MPEG, Xiph...) creates its own or copies it from another vendor.
Multichannel recordings are often referred to as "Surround".
rjamorim prepared some test streams so that you can check if your surround setup speakers are positioned correctly. The AC3 and DTS streams follow the standards set by the developers of such technologies (they were encoded with official encoders). The AAC stream was created with Compaact!, and it's impossible to guarantee that all channels will be decoded at their correct places when playing back with different AAC decoders. If you can, test the AAC stream with Compaact's "official" players - Mpegable DirectShow Decoder or Player.
In this test, a synthetic voice will say out loud the position where each stream should be coming from.