Different wikis have different communication systems in place, but typically, you would leave a note on the talk page associated with the article.
You can get to the talk page by clicking the Talk link next to the title of the article / at the top of the page. Help:Talk pages explains how to use talk pages if you are unsure.
Note: some wikis use the Article Comments feature which replaces talk pages on articles. In that case, you could leave a comment at the bottom of the page.
When reverting their edit, you can also leave a note in the edit summary, something like: "reverting edit -- please see my note on Talk:About" (you can put links in edit summaries)
If you want to talk directly to the user, you may also wish to leave a note on their User talk page (or Message Wall, for wikis which have that feature enabled).
There's a comment field in the edit window. Comments will be saved in the version history. See Help:Editing. Other than that, article talk/comments per above.
What do you think?