A type of Denial of Service attack launched against message boards (i.e. Usenet) and/or chat services.
The fundamental strategy is to post large amounts of useless, meaningless, or disruptive material. This has to continue for long enough to exhaust the patience of the regular users, forcing them to leave because of their inability to have a meaningful discussion.
There are many tactics that can be used while crapflooding: a massive campaign of old school trolling, cross-posting flamebait to an ideologically opposed group and letting the inevitable holy war erupt, or simply arranging to automatically post gibberish on a regular basis. Usenet advertising and porn spam is an excellent example of crapflooding in action, despite the advertisers only caring about the users as potential customers, instead of as targets. See also Slashdot troll.
Methods used to avoid crapflooding include killfiling suspected trolls, actively tagging relevant posts to prove their validity, and moderation (including automated systems such as Slashdot's Karma system). None of these systems is foolproof against a sufficiently motivated and organized crapflooding campaign, and many have the potential to cause significant problems within bulletin boards themselves.
The term also describes a tactic used on old dial-up bulletin board systems with upload/download ratios in place. Users attempted to upload random data or obsolete files, under misleading filenames, to gain enough credit on the board to download the files they wanted.