Angus Croll takes the reader step-by-step through some basic JavaScript code explaining what 'this' is in each step and why. He explains how 'this' depends on the execution context, and how it is manipulated by how JavaScript functions are called (or applied or bound with bind), and how Constructors create a new context.
Tim Caswell describes the JavaScript variable 'this', which is about current scope and current context. The only way to create scope in JavaScript is through function definitions, and in most cases the context is the receiver of the message (the object before the dot in the method call). He also talks about approaches to controlling what this references with call, apply and bind.