A simplified explanation of closures by Morris Johns: A closure is the local variables for a function - kept alive after the function has returned.
A stackoverflow community article explaining closures in plain simple English does a great job. Succinctly: a closure is created when inner function that gets returned. The inner function can still see the variables and methods defined in the outer function.
Douglas Crockford's presentation on Advanced JavaScript. He covers topics such as inheritance, modules, debugging, efficiency and JSON.