There is a list of global rules, as well as some board-specific rules [1]. Generally boards just require you to stay on-topic (unless you're on /b/, where the only off-topic thing is ponies, which is the only on-topic thing on /mlp/), refrain from spam, and not post anything that could get 4chan in legal trouble. Rule offenses result in IP bans, usually for a certain time (from a day to month to forever).

All of the boards have mods of various rankings (people who delete off-topic posts vs people who actually ban/temp-ban users) and various degrees of moderation. According to the post (and agreeing with my personal experience) moderation has generally been on the rise as of late, although apparently this is due to better moderation tools rather than an increase in moderators.

Prior to the new ban-list, most moderation (I've seen anyway) is pretty quiet, save from the infamous USER HAS BEEN BANNED FOR THIS POST that you sometimes see.

[1] https://www.4chan.org/rules