Please see the MediaWiki User's Guide to page history on meta for full details on this feature. For example, this User's Guide shows how to view the successive revisions to a page.
The page history of almost all old versions of pages is available. It consists of the text, the date and time of every edit, the user who made it, the edit summary, and a possible marking as "minor edit". is linked to from Recent Changes, Related Changes and from every article. The page history of deleted pages is not available to non-administrators. However, the revision history is kept for some time and can be retrieved by a sysop, who can also undelete the page. The history of uploaded files is also kept. The image history listing forms part of the image description page, along with the old versions. Images cannot be undeleted.
Special:Export can be used to download the source text of the current and old versions of a page, with the info in the page history lines. See m:XML export.
Archiving talk pages in separate pages is superior to using the page history as archive as they can then be searched and organized.
Wikipedia does not give legal advice.
The Wikimedia Foundation wishes to make users of historical versions of articles aware that some of the history may be problematic.
Title 17 United States Code § 108 (http://www4.law.cornell.edu/uscode/17/108.html) archive notice: page histories should be considered not for profit archival material. Although all contributions are supposed to be compatible with the GFDL, it is possible that a user has inserted something which is a violation of copyright, in which case the user had no right to release it as GFDL. In these cases, Wikipedia does not have a general policy of always deleting all copyright problems from the history, particularly if the edit is made to an existing article rather than a new one, although such material will be removed from the current version of a page. In the event of a DMCA takedown notice, the Wikimedia Foundation may remove violating versions from the page history. In other cases, the version may be kept to allow proper tracking of authors and demonstrate compliance with all legal requirements. You are being given notice under Title 17 that you are responsible for complying with copyright laws in regards to archival material, and that not all versions in a page history are necessarily available under the GFDL.
As noted at the end of each history revision, in the text at MediaWiki:history copyright, the page histories may contain material which has copyright limitations on them. Whilst we attempt to remove such material from the current version (see copyright problems) it is kept in the history for research and author attribution purposes. As Wikipedia is a not-for-profit company, this is believed to be fair use.
There is no ability in the current Wikipedia software to delete or hide specific versions of articles in the page history, though developers have the technical ability to do so, via editing the database directly.
See also Wikipedia talk:Copyright violations on history pages for further discussion of this point.