The page history of almost all versions of pages is always 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". It 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.
Archiving talk pages in separate pages is superior to using the page history as archive as they can then be searched and organized.