This document is for non-janitor monks who are curious about what this shadowy cabal known as janitors actually do. If you are a janitor yourself, please read Janitors' Guidelines and Janitor Powers for instructions on how to do your job.
The janitors (who are also, in some docs, called "editors") can:
One of the main thing Janitors are entrusted to do is ensure that node titles are meaningful and useful for the site. (See How do I compose an effective node title?) "In a very real and legally binding sense," node titles have a dual ownership between the node author and the site janitors, unlike node content. A poorly written title can greatly hinder site navigation. For example, one-word titles tend to be a problem, since they can look like keywords and usernames; and misspellings grossly impair searchability. Janitors are authorized to fix any such problems as they see fit, while respecting, to the extent possible, the intentions of the author.
Janitors can edit the actual content of a node, but in general, they will only use this power to clean up formatting problems that cause the node to fail to render usefully in most browsers. Modifying otherwise usable markup to make it compliant with some standard (such as HTML 4.01) is not in their charter.
Janitors are specifically directed not to use their power to fix typos, add or delete information, or correct factual errors. In general, you should also not ask them to linkify URLs.
They are licensed to delete email addresses and passwords. When a post contains material of an objectionable nature, such as incendiary troll trash or links to pirated material, the entire post is usually considered for reapage and that process does the job.
...for example, when someone puts the whole post inside <code> tags.
That's really the janitor's judgement call.
On the one hand, it could be left to the author to fix, and a /msg to the author is therefore the best course of action.
On the other hand, a janitor may feel that cleaning up a poorly formatted node, as a service to those who have to read such a node, and as a way of illustrating how nodes ought to be formatted, is reasonable.
And if the node's author is Anonymous Monk, it can't be fixed by the author, so if it's going to be fixed at all, it's got to be done by janitors.
The changes that janitors make are automatically logged by the PerlMonks engine. All Monks can view the changes by appending &displaytype=edithistory to the URL; monks of level Friar or above can also do so by clicking node history in the Approval Nodelet.
The janitors don't necessarily all agree on the exact scope of their priorities and prerogatives. Each one tries to do what seems right and proper and best for the Monastery, within the constraints of conscience and tools. If you have questions or issues, you can raise them in the chatterbox; there are usually a few janitors and gods around. You can also post in Perl Monks Discussion.