Forum wars are thread wars that usually consist of a forumer posting a thread in a not-liked forum saying their forum is attacking. It ends with big amounts of spam threads and bans, due to forum moderators not being fond of them.
Forum wars are usually the Let's Make a Deal forum VS the Clans & Guilds forum due to those two forums thinking they are better than each other. The posts from C&G are usually "We know how to fight and you guys love your pixels" and the posts from LMaD are usually "We can buy C&G".
Lesser-known forum wars are usually against the Roleplaying forum, oftentimes criticizing it due to the behavior of players in roleplaying games, rather than the far more professional roleplayers in the forum.An example of someone starting a forum war: