Greasemonkey is a browser extension that enables users to customize how Web sites display on their computers. Once Greasemonkey has been installed, user script s enable changes to the elements of Web pages. A user script may be either general or specific to a particular site. For example, a user script for Google called "Butler" makes it possible for users to remove advertisements and add links to Google search results.
Greasemonkey was created for the Firefox browser ; subsequently, versions have been developed for other browsers, including a version for Internet Explorer called GreasemonkIE. According to some analysts, the ability to alter online content may lead to the so-called hacked Web , a situation in which much of the Web is open to modification by users.
Greasemonkey and user scripts are relatively easy to install and therein lies a security issue. An unscrupulous programmer could insert malicious programming into a user script, and the average person would not be able to detect it by looking at the code. Malicious user scripts, like other malware , can harm a host computer or steal sensitive information, among other things. Aaron Boodman, the 26-year-old Seattle programmer who created Greasemonkey, suggests that user scripts should be approached with the same caution as other software, and that people should ensure that they come from a trusted source before installing them.
In the real world, "grease monkey" is a slang term for a mechanic.