Full history is kept for every page in the repository. You can view that history by clicking on the "History" link at the bottom of the page. There you can see the time, author and comment of each edit. By clicking on the date, you can see the actual changes made.
Clicking on the "Undo" button next to a change will rollback that change, and all later changes, to the state the page was in just before that edit. This is useful for reverting spam.
If you undo the very first change, the page will be deleted.