There is much confusion regarding so-called "Ogg files." Due to the rising success of Ogg Vorbis, many people have begun to use the term "Ogg" as a synonymn for the Vorbis audio compression. This is incorrect.
Ogg is a container format. Container formats themselves do not contain data, but instead define a framework in which data can be held. The container itself identifies and seperates the different types of data that it holds so that they can be manipulated and used.
The Ogg container format can hold a variety of different types of data. It supports audio and video, in both compressed and uncompressed formats. It also allows for subtitles and captions.
The Ogg container format is not restricted by software patents, and when combined with a Free Codec such as Vorbis, it can be used to create fully Free multimedia.
Some codecs that are commonly used in Ogg containers:
Vorbis -- compressed audio, designed for general audio and music
Speex -- compressed audio, optimized for voice data
FLAC -- uncompressed audio
Theora -- compressed video