Making a popup and greying out the rest of the screen

Hi all,
I've seen a new trick used alot on the web and its very effective. Im notsure what its called but it does the following:
- When a user clicks something a popup window or box appears in the center of the screen.
- The popup stands out becaue the rest of the page is greyed out and the page elements cant be clicked.
- To proceed and get back to the main interface, the user must click a button on the popup. E.g. close, confirm, cancel.

I've been trying to find an example but cant remember where i saw it.
Is there a component or something to do this type of thing?
Thanks!!
O