Closures are really powerful in javascript. The problem with them is that a lot of javascripters don't understand them well. When I first started working seriously with closures, I had so many questions and no answers… until I read JavaScript Closures for Dummies. Don't be fooled by the “for dummies” that could make you think about low-quality information as every “for dummies” books. The article of Morris Johns is detailed and precise… but lenghty.

Don't forget to it and use it as a reference!