The following Wikipedia-user-reference outline is provided as an overview of and topical guide to userscripts:
Userscript – userscripts are programs, usually written in JavaScript, for extending the capability of one's browser and for modifying web pages. Using them is referred to as augmented browsing, and there are thousands of them available around the Internet. Those work through a browser extension, and some of them are designed to work on Wikipedia pages. Independent from those, Wikipedia includes a feature that allows users to install user scripts (designed specifically for use on Wikipedia) on their Wikipedia accounts, and those only work while a user is logged in on Wikipedia. Within the Wikipedia community, scripts designed to work via WP user accounts are referred to as "user scripts" (notice the space between "user" and "scripts").
This outline was written mostly with the JavaScript programmer in mind. It attempts to track down the source code for all userscripts (external and internal, including gadgets), and everything about developing user scripts (including gadgets) for Wikipedia and MediaWiki. MediaWiki is the program that Wikipedia is run on.
Note that most gadgets are JavaScript programs (the rest are css snippets). Gadgets are selected by user account, and therefore, those that are JavaScript programs are user scripts, though they are removed from user script lists once they become "gadgets". The type of gadgets that are JavaScript programs are covered by this outline.