need help with creating an overlay, example website included... Options

[AlphaFreak]

Hello there!


i'm building a website at the moment which has a login part to it. I am trying to create an overlay so that when a link is clicked, a form will appear over the page. and the rest of the website will turn a shade darker.

here's an example of what im looking for:

http://kissmetrics.com/


Top right hand corner theres a link to login.

How would i go about creating something like that? what kind of code will i need to use?



Thanks in advance!


~AF