Each time a page is changed, a new version is saved and the old one is moved to the attic.
Old revisions of a page can be listed by pressing the Old Revisions button.
The diff function is available from here, too.
Where the files will be physically stored in the system is configured through the savedir and the olddir option.
The default system path is:
DokuWiki_installation_path plus data/attic plus (optionally) full_namespace_path plus timestamped_page_name.
The full contents of the wiki source are stored here.
The compression configuration option specifies if the pages will be saved as compressed files or not.
To remove any of the contents of the attic you can manually remove the corresponding files and sub-directories from the system.