A bug is a mistake in the programming of the game. They are problematic because they can, for example, cause your fleet to disappear or show incorrect production values of your Fusion Reactor. Most bugs are simply aesthetic and cause no damage, however, a bug can theoretically do anything. Reported bugs are usually fixed when there is a new update to the game.
Users have to report bugs which they have discovered quickly, so that it can be fixed quickly. Users who don't do this can be locked because of Bug using according to § 2 of the rules.
Bugs shall be reported in the OGame board under Bugs & Complaints, in order that the administrators can find them easier and faster and fix them through the programmers. Since there are usually many users affected by a bug it is recommended do post ones message into a clearly titled collective thread instead of a common frequented Thread intended for Complaints concerning the game or similar ones.