Rollback quickly reverts all consecutive edits by the most recent contributor to a particular page. The edit summary is automatically set to read "Reverted edit of A, changed back to last version by B", and the edit is marked as "minor".
The rollback tool should only be used to revert blatant vandalism. It should not be used to revert good-faith edits, to resolve content disputes, or to enforce a particular version of an article.
The rollback tool may remove constructive edits, and it may also leave earlier vandalism intact. As a result, after using the rollback tool, sysops should always check recent revisions and ensure that all disruptive edits have been reverted and all constructive edits restored.
Finally, after using the rollback tool, sysops should leave a note on the appropriate User talk: page and explain why rollback was used. Many "vandals" are simply new users who are unfamiliar with the ways of the wiki; by gently correcting them, a good sysop may lead them to contribute positively.
If you are an administrator you can use bot rollback, which will treat rollbacks as bot edits so they don't flood recent changes, good if a vandal bot comes and vandalizes lots of pages. To use this, you need to go to the user's Special:Contributions page and put &bot=1 at the end of the url, like this, http://en.wikipedia.org/w/wiki.phtml?title=Special:Contributions&target=SomePersistentVandal&bot=1. You can then click the rollback buttons on the user's contribution page so that the rollbacks don't flood recent changes.