Click to insert the sample code at the current cursor location.
The output pane is just a div (called #output). There are helper functions log and logn (print line) which will simply append some text - nothing to do with coffeescript.
Coffeescript is "JavaScript's less ostentatious kid brother"... you can assign values to variables and create functions with parameters. You can iterate over lists and do nice pattern matching stuff.
And oh-so-much more... there is a simple class system, useful while loop, splats... I'll add more soon.