Browsers allow you to save not only URLs, but also scripts as bookmarks (Firefox, Safari) or favorites (Internet Explorer). These special links that contain scripts are known as bookmarklets, or favelets.
To save a bookmarklet, just do like for regular bookmarks: right click on the link, and select "Add to Favorites" (Internet Explorer) or "Bookmark". Because the link contains code, you'll get a warning. Only accept bookmarklets from trusted sources! Your bookmarklet is then ready for use.
When you select a bookmark, the browser loads the corresponding Web page. When you select a bookmarklet, the browser remains on the current page and runs the script.