Photoshop :: Billboard Graphics

I'm talking about the gigantic full color signs that we often see along the highway, Billboard Signs.

I've seen them for advertising for cars, radio stations, soups, pretty much everything.

I'm wondering, what program do they use to make something of that size?

I mean, surely it's not as simple as taking an image into photoshop and creating whatever wording you want on it; then telling it to print to a massive printer.

I actually have a task set before me to design such a thing,