Help:Page history

An exception are pages which have been deleted (not to be confused with paged that have been blanked), the only record available for non-administrators is the deletion log and possibly the "votes for deletion" archive. Edits to them are not even in the User Contributions pages. However, the revision history is kept and can be retrieved by a sysop, who can also undelete the page (see en:Wikipedia:Viewing and restoring deleted pages by sysops.
The policy of keeping the page history did not exist in the beginning of Wikipedia's existence, so the page history of that time is lost.
Every page has a link "Page history" to the list of edits. Also the Recent changes and Related changes pages have links to the histories of listed pages.
Every line represents one edit to the given page and the version resulting from it
[list][*]"last" is similar to "diff", explained in Recent changes
[*]"cur" gives the difference between this version and the current one (i.e., excluding the edit shown by pressing the "last" next to it), which is the cumulation of all later edits, including those which are not in this revision history because they were made after loading this page
[*]the date and time link to the version of that day and time, except that in the first line, which is about the latest edit at the time of loading this revision history, the date and time link to the current version, hence with the changes made after this revision history was loaded;
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The "cur" and "last" features are similar to those in Enhanced Recent Changes, except for "cur" in the first line: it is not linked in the revision history, while in the Enhanced Recent Changes it gives the differences corresponding to the last edit; just after loading the revision history this is the same as the "cur" link of the second line gives, but if there have been edits after loading it is different.
For every version there is a box on the line. Marking two boxes gives the difference between the two versions. The box at the top corresponds to the latest edit, which is not necessarily the edit about which the info on the line is (another edit may have been made after loading the page history),
If the "move page" feature has been used in the past to change the name to the current one, the entire edit history of the article, before and after the move, is shown, but the move itself is not. The move itself is shown in the edit history of the page with the old title, which automatically has become a redirect page.
See also: en:Wikipedia:Usemod article histories.

Through Export the source text of the current and old versions of a page, with the info in the page history lines, can be packed together in a single XML-file. Also for a list of pages all can be put in a single file. This also allows searching for a text in the collection of versions. See also XML export.

An "image" (in the broad sense of an uploaded file) can be edited, or, more generally, be replaced by a different image, by uploading a new image file with the same name. Again all versions are kept. The image history listing forms part of the image description page, which appears when clicking on the image. The image history consists of this and the old versions themselves.
The image description page consists of three parts:
[list][*] description of the image
[*]"Image history"
[*]"Image links"
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The description part has also an edit history; it is labelled, somewhat confusingly, Image:xxx Revision history. It should not be confused with the Image history.
Not kept are images which have been deleted (not to be confused with images that are no longer used in articles), the only record available is the upload log, deletion log and possibly the "votes for deletion" archive. Neither the latest nor older versions are kept by the system, hence it is not possible to undelete an image.

The actual history of an article with images is the combined effect of the revisions of the source text, including references to images, and the revisions of the images themselves.
As a result:
[list][*]Old versions of articles do not show corresponding old versions of images, but the latest ones, unless the file names of the images have changed.
[*]Changes in images are not shown in the page history of the article if the same image name was used.
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Similar remarks apply for templates.

Archiving texts in separate pages is superior to using the page history as archive: texts in archive pages can be found searching with Google and using the search feature of Wikipedia. Page histories can only be searched after applying Export. Also, archive pages can be organized and titled afterwards in a suitable way, while e.g. edit summaries can not be supplied afterwards. However, an index of old versions of a page, with links to them, could be prepared.