HotCat is a gadget that helps registered users easily remove, change, and add categories to Wikipedia pages. It has a suggestions list that will propose existing categories for auto-completion.
To turn HotCat on or off, first make sure you are logged in. Then, click on "Preferences" in the upper-right corner, go to the "Gadgets" tab (or use this link), and scroll down to the "Editing" section. Check the box near HotCat to enable it. Scroll down to the bottom of the page and click Save. You will get a "Your preferences have been saved" message. You can now navigate back to the page you started from; you may need to reload the page you were viewing or bypass your cache.
To disable, the instructions are similar to enabling it except that the box near HotCat has to be unchecked.
Whenever a page is loaded, HotCat looks for the category bar. If there are visible categories, it adds links to easily remove, change or add categories. Example:
If HotCat can detect that a certain category is not in the page text itself but was transcluded onto the page from a template or other transclusion, it will not add these links. Only categories that are present in the wikitext of the page itself can be modified through HotCat.
To remove a category, simply click the "(−)" link next to the category name. If the page does indeed contain this category, HotCat will remove it automatically. It will: