Stereo refers to audio streams with two channels. It's by far the most popular setup these days, given it's the standard mode for audio CDs. Several radio and TV broadcasts are also stereophonic.
In the past few years, there has been a push from the recording industry and equipment manufacturers trying to convince customers to move from stereo (CD) to multichannel (SACD, DVD-Audio). Reaction to these efforts have been lukewarm, and that is credited to the fact that the vast majority of the consumer base is "happy enough" with CD.
The channel order in stereo is left-right.