As a client-side scripting language (as opposed to a server-side scripting language, like PHP, ASP, and Perl) it runs within the browser itself on the visitors computer, rather than being pre-parsed by the server, and then passed to the client. This means that all the work is done by the web visitor's browser after having been downloaded, rather than the web server that the code resides on.

So why would we want to do use this? Why not just do everything on the server side, rather than taking the chance that the client might have javascript disabled on their browser?

The answer is that we can provide a measure of function to a page that can only be duplicated by using even more complex, expensive tools (such as Flash), or by forcing a page reload so that the server can do further work for the client. With Javascript we can do such things as random/cycling w/out a reload, onMouse events, auto-launching objects and actions, the ever popular pop-up and pop-under windows, etc.