CoffeeScript is a little language that compiles into JavaScript. The code compiles one-to-one into the equivalent JS, and there is not interpretation
at runtime. You can use any existing JavaScript library seamlessly (and vice-versa).
The syntax of CoffeeScript is inspired by Ruby and Python, and implements many features from those two languages. This introduction references The Little Book
on CoffeeScript and the offical site of CoffeeScript heavily.
You are required to be familiar with JavaScript’s internals and idioms, and Ruby’s and/or Python’s syntax.
For more details, check out http://xjia.heroku.com/2011/08/08/a-brief-introduction-to-coffeescript/