Documentation for User:Ais523/catwatch.js:

This script can be used to monitor categories of your choice for changes. Due to technical restrictions, the script will only report additions to the category, not removals. To be precise, for each category you're watching, it will show, on your watchlist, the page which was most recently added to the category, as if an edit to the category, at the time it was added. (However, the script cannot link to the edit in which it was added; a link to the most recent edit on the page in question is provided as a convenience, although it may be an entirely different edit.) Previous versions of this script also erroneously reported recategorisations and changes of interwiki links, for technical reasons; this is no longer the case, due to changes in the MediaWiki software that powers Wikipedia.

In order to determine what categories you're watching, you will have to make a separate category watchlist, created by the script the first time you run it, and from then on accessible via Special:Mypage/WatchedCategories.js. Note that this page cannot be edited by other users, but is visible to them; thus, other people will be able to see which categories you're watching. You need to bypass your cache after editing this watchlist, for the edits to take effect.

The format of the watchlist is a mandatory header and footer to make it valid JavaScript, plus a comma-separated list of quoted strings, each of which is the name of a category (without the Category: prefix). For a comparison, here's mine. Make sure you get the case correct; page names start with an initial uppercase letter, and all the rest are case-significant, and this script cares that you get it right.

This script is known to be somewhat buggy in some versions of Internet Explorer; if it seems to be acting strangely to you on such a browser, try to completely clear your cache (instructions at WP:BYPASS) rather than just bypassing it, and the problems tend to go away.