All users are able to revert an article to an earlier version, but users with rollback permission, administrators and bureaucrats have a fast, automated tool to help them. This is rollback, which can be accessed from Special:RecentChanges and the page history.
Rollback reverts all edits by the last contributor to the page to the last version not authored by that user. For example, if user "Foo" makes six bad-faith edits and rollback is clicked, all six edits will be reverted until the software reaches the last version that was not authored by Foo. When rolling back, the automatic edit summary format "Reverted edits by X (talk | block) to last version by Y" is used.
If a page is only authored by one user, rollback is impossible.
When to use
Since rollback does not give the option to supply a custom edit summary, it is only to be used to revert spam or vandalism, as good-faith contributors may be deterred if their edits are rolled back. It is also preferable to view the difference between edits to verify that it was indeed a bad-faith attempt.
I hope this tutorial was helpful!