Greasemonkey is like an add-on that makes your browser extensible in yet another way.
These scripts run on web pages and can often add features that are not available by default. For example, you will find Greasemonkey scripts that add a download button to YouTube, preview audio files stored in Google Docs, or show prices in your local currency on Amazon. Since these scripts run in the context of the website itself, many of these can be run on any browser, of doing so. The great thing is that Greasemonkey scripts are natively supported on Google Chrome, without installing any add-on.