JavaScript is the primary method for running code on the client's browser.  JavaScript is a very powerful tool when developing for the web.  It is one of the basic building blocks of DHTML and is a primary building block of almost every web site out there.

JavaScript allow you to run code to manipulate web pages with code run on the clients browser.  Back in the day, JavaScript became the defacto standard, beating out propriterty VBScript, for running client side code, and now is supported in every browser worth a damn.

Writing good JavaScript isn't quite as hard as it once was.  Back in the days of the Version 4 browsers, Netscape and Microsoft had very different ideas of how JavaScript (HTML for that matter) was to behave.  The rules have gotten a little better with the wide acceptance of the W3C standards.

JavaScript is quite powerful for manipulating HTML via the Document Object Model (DOM).  You can basically create an entire webpage on the fly using client site JavaScript.