Banned users are people who have been formally forbidden from editing a Wikimedia wiki. This designation can be temporary or permanent.
Different wikis have different policies for banning users. Typically users are banned for personal attacks, spamming, legal threats, chronic vandalism, or other forms of disruption.
A ban is sometimes enforced by a block, a technical measure which prevents a particular user, IP address, or range of IP addresses from editing pages. Note that bans and blocks are not synonymous: a ban is the result of a formal decision or substantial community consensus, and generally results from a pattern of misbehavior, whereas any administrator can enact a block for simple vandalism or other clear violations.
Banned users wishing to rejoin the community should go about the appeal methods usually set down in their ban decree. If such a condition is absent, one can usually take the issue (on a Wikimedia-run Wiki) to one of the mailing lists, Jimbo Wales himself, or possibly IRC.