Dolby Pro Logic
Dolby Pro Logic is the surround sound codec that really put surround sound for home theater on the map. Powered by the boom in popularity of VHS in the late 1980s, Dolby Pro Logic brought a reasonable, non-discrete surround track to people's living rooms. It's fair to say that Pro Logic gave people reason to invest in their first home theater receivers, AV preamps or even standalone surround sound processors.
Dolby Pro Logic is a matrixed format. This means that there are two stereo channels (on the VHS tape) and within these two channels exists the data to create a center channel and single rear surround channel (which is split by the receiver to two speakers). The advantage of this method is that non-Pro Logic devices (a television, say) can still read the stereo data, and not have any issues because doesn't "see" the matrixed information. The downside is a lack of truly discrete surround or LFE channels.
In 2000, Dolby released Pro Logic II, which creates 5.1-channels from 2-channel stereo signals. Though not nearly as good as a true discrete soundtrack like you'd get from Dolby Digital or DTS, PLII is still very popular to get all the channels going in a surround sound system with music or stereo movie.
The latest incarnation is Pro Logic IIz, which creates height channels.
While most AV preamps can decode Dolby Pro Logic today, it is not considered the best format for surround sound, as there are both lossless and lossy codec, such as Dolby TrueHD, DTS Master Audio and even Dolby Digital that do a better job with surround effects, specifically providing less compression to the sound and allowing discrete audio for each of the 7.1 channels in a system.