How do CPM ads work?

This might be a newbie question (in fact, I know it is) but how exactly does CPM advertising work? Do you get banners to put on your site and you get paid for everyone who visits?

You get paied for each 1.000 banners that you display to your site.

The rates are very low in the most cases, but it is good for sites that have hundrends of thousand of pageviews every month.

Ok, thanks. I'd heard it was using popups and popunders more than anything else, so I just wanted to confirm that there are banner ad programs for it. :)

There are many formats for CPM based ads including banners, popups, popins, interestials, etc.

But the big networks using mostly banner advertisments.

Try to refrain from using popups.  Your visitors will hate them.

Yeah, popups were never even a consideration haha. I was thinking of using CPM if I ever manage to grow my forum, and wanted to do it with banner ads. :)