A container format is a file structure to combine audio/video data, tags, menus, subtitles and some other media elements (e.g. seeking ability, chapters...). Flexible containers can hold any type of music file, for example you could put an MP3 inside an Ogg container, although that would probably just lead to confusion...
The most popular containers are:
- AVI - the standard Microsoft Windows container
- MOV - standard Apple MacOS container
- MP4 - standard container for the MPEG-4 multimedia portfolio
- Ogg - standard container for Xiph.org codecs
- ASF - standard container for Microsoft WMA and WMV
- Matroska - not really standard for any codec or system, but still popular in the digital video scene