Hi,
every edit you make is logged. That means, by editing an article you create a new version of it, but the previous versions are still available for everyone. That also happens if you remove all content of an article. Some users patrol the recent changes on Wikipedia using several tools and check if the edit is ok or if the edit is bad in any way.
Since all versions are logged, the patrolling user can now edit the previous version and save it again - and so revert the edit. Therefor, most users use tools that make reverting even easier and faster. On the English language Wikipedia there is also an "anti-vandal" bot that checks new changes automatically and reverts them if it detects vandalism (if you empty a page for example, the bot would immediately revert that).
You see, there's no way for you to delete an article completely. Only the 1,200 administrators on the English Wikipedia are able to do so. But even their deletions are not ultimate. Other administrators can still view the deleted versions and, if they think it is necessary, can restore them. There are only some 30 users who can delete permanently so that only the server admins could restore.
I'm administrator on several Wikis myself, I hope I could help a bit.