Though the MP3 format has become very common, it is still owned. Every time a distributor sells or gives away music encoded as an MP3, they must pay a fee to the owners of the MP3 patents. The patents are also an issue for developers writing software to work with MP3s. In contrast, OGG files – short for Ogg Vorbis – are in the public domain, so anyone can use the format or write software to use it without being dependent on a patent holder for permission.
The campaign is aimed at distributors and listeners. Many personal digital music players support OGG files, and there are free software players, encoders and plugins available for a variety of operating systems. FSF predicts that OGG has the elements needed to surpass MP3 in use.
As part of the campaign, FSF will be promoting Ogg-friendly websites, providing instructions for installing and using Ogg Vorbis, and a directory of volunteers offering their technical expertise to sites seeking help in making the switch to free formats.
Pretty much everything you could ever want to know about OGG can be found at the PlayOGG website.
Technical details about the OGG format can also be found at Xiph.org