What is an "overlay"?

[Tom Vaughan]

Hello,

Could someone please tell me what an "overlay" is? Or point me at some
documentation on this? I've looked through the docs at mozilla.org, the xul
tutorial at xulplanet.com, and yet another xul tutorial, and I'm still
confused. Thanks.

-Tom

-- 
Tom Vaughan <tom at vaughan dot to>