An edit war happens when people do not agree on an article's content. Instead of discussing it on the talk page, they keep changing the article back and forth. If this happens, an administrator may protect the page for a few days so that no one except administrators can change it. This is so that everyone will have a chance to think about the article and come to a consensus or compromise.
It is not a good idea to start or join an edit war. The Wikipedian way is to talk about it and have all sides give their opinion. If that does not work, ask a Wikipedian who has not been working on the article to give their opinion.
Three revert rule[praviti]
Because edit wars go against the idea of consensus, users who take part in an edit war can be blocked. This usually happens if the user reverts (changes the page back) three times in a day. If someone is in an edit war, an administrator can block them even if they have not reverted the page three times. If a user is reverting obvious vandalism, then the three revert rule may not apply to them.