A script language (system of programming codes) that was created by Netscape. Javascript can be embedded into the HTML of a web page to add functionality. Currently, the majority of search engine spiders have difficulty in executing JavaScript.  However they are getting better.

From an SEO point of view, there are still problems associated with the inclusion of Javascript so depending on your needs, you may want to limit the use of JS to areas/pages where SEO is less of a concern.