Tools, part 1: References, external links, categories and size
Broadly defined, a tool is any implement designed to make a process simpler, more efficient, or easier, adding extra functionality in the process. On Wikipedia, tools for editing articles run in various ways; some are attached to the standard edit toolbar (which may need to be activated in the Preferences), some to the sidebar, some are bots, and others run on the Toolserver. Some are completely independent of Wikipedia and run on their own, separate sites. However, they all have something in common: they were designed and built by competent coders to make editing easier and faster.
User scripts consist of JavaScript code that runs in your browser, adding functionality to Wikipedia's standard user interface. They range from extra links to extensive editing platforms. Never copy scripts into your skin.js page; instead use importScript("User:Example/awesome script.js") which keeps scripts updated with bug fixes and enhancements. Compatibility varies with skin and browser, with Internet Explorer being problematic. A more extensive script list is at Wikipedia:WikiProject User scripts/Scripts.