What does Client Side Javascript actually consist of? Any book recomandations for studying it?

In my quest to master JavaScript I have already gone and studied the JavaScript core defined by the ECMAScript specification and am now ready to move on to Client Side Javascript.

My biggest problem is that I cannot find a book or online reference that has an overview on what Client Side Javascript is. Looking online at the Mozilla Web Docs reference I came across a tutorial on Client-side web APIs.

My question is: Does client side JavaScript equal Client-side web APIs or is there more to it?