What's a Wireframe?

Occasionally, I am in the middle of a conversation, and the listener stops me and asks..."what's s a wireframe?" (Most of my conversations involve wireframes sooner or later.)

Here is an explanation I wrote last year that includes pictures of famous cartoon characters:

Tigger, Pooh & Judy Jetson Meet Wireframes & IA

Tunes: Killers: Mr Brightside