Descrition

Ogg is a digital media container format, developed by Xiph, that can encapsulate various bitstreams, for audio or video codecs, arbitrary data (e.g. subtitles) or metadata. Its first purpose was to be the native container for the free codecs developed by Xiph, and it was developed alongside with the Vorbis codec.

Features

encapsulation and interleave of multiple data streams,
packet framing
error detection
seeking

All in a small, bounded percentage bitrate overhead.

Also, Ogg format specification is now published within the IETF in the following RFCs:

RFC 3533 The Ogg Encapsulation Format Version 0
RFC 3534 The application/ogg Media Type

Main uses

Here are the typical bitsreams contained in Ogg:

Audio

Vorbis for general lossy compression (aka Ogg Vorbis, the first and classical use of Vorbis)
Speex for voice lossy compression
FLAC for lossless compression

Video

Vorbis sound and any video codec, usually MPEG-4 codecs like DivX or XviD (aka OGM, Ogg Multimedia)
Vorbis sound and Theora video, using only free and patent-clear (as claimed) formats from Xiph (aka Ogg Theora)