Side-channel speakers are what took 5.1 to 7.1 surround. 7.1 surround consists of speakers in the front left and right, a center speaker, rear left and right speakers, as well as a side left and right speaker, along with a "point 1" subwoofer. Another option is 6.1 surround, which uses just a single rear speaker.
Purists would suggest your side-channel speakers should be the same type of speakers as your front speakers, but that often doesn't fit the room and/or budget requirements for home theater enthusiasts' systems. Side-channel speakers help make the surround sound effects more encompassing and three-dimensional. It is perfectly acceptable to use smaller speakers or even in-wall speakers as side-channel speakers in a home theater. It is recommended that, if at all possible, your side speakers are the same variety as your front three speakers to keep a uniform sonic flavor throughout your system.
You'll need a receiver or preamp/amp combo to get surround channels, plus a source like a Blu-ray player to make full use of side channel speakers. Dolby TrueHD and DTS-HD Master Audio both have 7.1 channel options.