Top Google result for "javascript this": http://www.quirksmode.org/js/this.html
Edit: I think the key sentence is:
"In JavaScript "this" always refers to the “owner” of the function we're executing, or rather, to the object that the function is a method of."
Quirksmode is (generally*) excellent, worth reading the whole article in detail.
*Apparently this statement is not necessarily correct, see T.J. Crowder's comment below.
