(Copy edit per MOS:MDASH, etc.)
[[Wikipedia:Reverting|Reverting]] is reversing a prior edit, in whole or in part. Revert [[wp:vandalism|vandalism]] upon sight but revert an edit made in good faith only after careful consideration. It is usually preferable to make an edit that retains at least some elements of a prior edit than to revert the prior edit. Furthermore, your bias should be toward keeping the entire edit.
==Reverting drives away editors==
Reverting tends to be hostile, making editing Wikipedia unpleasant. Sometimes this provokes a reciprocal hostility of re-reversion. Sometimes it also leads to editors departing Wikipedia, temporarily or otherwise, especially the less bellicose. This outcome is clearly detrimental to the development of Wikipedia. Thus, fair and considered thought should be applied to all reversions given all the above.
The main purpose of reversion is to undo vandalism. If you see an edit that you're sure was intended by its author to damage Wikipedia, and it does, there is no need for further consideration.
In the case of a good faith edit, a reversion is appropriate when the reverter believes that the edit makes the article clearly worse and there is no element of the edit that is an improvement.
Whenever you believe that the author of an edit was simply misinformed, or
Another kind of acceptable reversion is an incidental one. A Wikipedia editor is not expected to investigate the history of an article to find out if an edit being considered is a reversion of some prior edit. The rule against reversions applies only to cases where the reverter is aware that the edit is a reversion of another edit.
There are a number of things that sometimes motivate an editor to revert, but
Even if you find an article was slightly better before an edit, in an area where opinions could differ, you should not revert that edit, especially if you are the author of the prior text. The reason for this is that authors and others with past involvement in an article have a natural prejudice in favor of the status quo, so your finding that the article was better before might just be a result of that. Also, Wikipedia likes to encourage editing.
Reversion is not a proper tool for punishing an editor or retaliating or exacting vengeance. No edit, reversion or not, should be made for the purpose of teaching another editor a lesson or keeping an editor from enjoying the fruits of his crimes.
Being reverted can feel a bit like a slap in the face
Explaining reverts also helps other people. For example, it lets people know whether they need to even view the reverted version (in the case of, e.g., "rv page blanking"). Because of the lack of [[paralanguage]] online, if you don't explain things clearly people will probably assume all kinds of nasty things, and that's how [[wikipedia:edit war|edit wars]] get started.