Auto Reverting Non-File Buffers
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Global Auto Revert Mode normally only reverts file buffers.  There are two ways to auto-revert certain non-file buffers: by enabling Auto Revert Mode in those buffers (using M-x auto-revert-mode); and by setting global-auto-revert-non-file-buffers to a non-nil value.  The latter enables Auto Reverting for all types of buffers for which it is implemented (listed in the menu below).

Like file buffers, non-file buffers should normally not revert while you are working on them, or while they contain information that might get lost after reverting.  Therefore, they do not revert if they are modified.  This can get tricky, because deciding when a non-file buffer should be marked modified is usually more difficult than for file buffers.

Another tricky detail is that, for efficiency reasons, Auto Revert often does not try to detect all possible changes in the buffer, only changes that are major or easy to detect.  Hence, enabling auto-reverting for a non-file buffer does not always guarantee that all information in the buffer is up-to-date, and does not necessarily make manual reverts useless.

At the other extreme, certain buffers automatically revert every auto-revert-interval seconds.  (This currently only applies to the Buffer Menu.)  In this case, Auto Revert does not print any messages while reverting, even when auto-revert-verbose is non-nil.

The details depend on the particular types of buffers and are explained in the corresponding sections.

Auto Reverting the Buffer Menu
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If auto-reverting of non-file buffers is enabled, the Buffer Menu (see Several Buffers) automatically reverts every auto-revert-interval seconds, whether there is a need for it or not.  (It would probably take longer to check whether there is a need than to actually revert.)

If the Buffer Menu inappropriately gets marked modified, just revert it manually using g and auto-reverting will resume.  However, if you marked certain buffers to get deleted or to be displayed, you have to be careful, because reverting erases all marks.  The fact that adding marks sets the buffer's modified flag prevents Auto Revert from automatically erasing the marks.

Auto Reverting Dired buffers
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Dired buffers only auto-revert when the file list of the buffer's main directory changes (e.g., when a new file is added or deleted).  They do not auto-revert when information about a particular file changes (e.g., when the size changes) or when inserted subdirectories change. To be sure that all listed information is up to date, you have to manually revert using g, even if auto-reverting is enabled in the Dired buffer.  Sometimes, you might get the impression that modifying or saving files listed in the main directory actually does cause auto-reverting.  This is because making changes to a file, or saving it, very often causes changes in the directory itself; for instance, through backup files or auto-save files.  However, this is not guaranteed.

If the Dired buffer is marked modified and there are no changes you want to protect, then most of the time you can make auto-reverting resume by manually reverting the buffer using g.  There is one exception.  If you flag or mark files, you can safely revert the buffer.  This will not erase the flags or marks (unless the marked file has been deleted, of course).  However, the buffer will stay modified, even after reverting, and auto-reverting will not resume. This is because, if you flag or mark files, you may be working on the buffer and you might not want the buffer to change without warning. If you want auto-reverting to resume in the presence of marks and flags, mark the buffer non-modified using M-~.  However, adding, deleting or changing marks or flags will mark it modified again.

Remote Dired buffers are currently not auto-reverted.  Neither are Dired buffers for which you used shell wildcards or file arguments to list only some of the files.  *Find* and *Locate* buffers do not auto-revert either.

Note that auto-reverting Dired buffers may not work satisfactorily on some systems.