What is DoubleClick and what are some good Ad networks?

Hi everyone, I've been in search of an ad network like Google's AdSense but with Flash and Picture banners, which can be very targeted at my site's users.

I found by right clicking on many ads around the net that their source was from ad.doubleclick.net, which I looked up but can't quite figure out what it is or how it works.

I thought I would ask here and get a human answer instead of contacting their support like they want me to.

thanks!

edit: Conveniently enough, both the ads on this page show up to me as flash ads from Google that say "Watch this space". So Google is analyzing the content of this page itself and serving an ad that is related to it, which is what I would like