What's an ad server?

Pretty much exactly what it sounds like.

The Google product for example is called

http://en.wikipedia.org/wiki/Ad_sense

Suppose you run a great website with a high number of visitors. An ad server like the one from Google will generate revenue for you (and for Google of course).

It’s a network of machines that seek to steal your bandwidth, slow loading of interweb sites, and is sometimes used by nefarious people to install malware through flash or similar browser exploits

there is no sane reason to allow ads to be downloaded- or rendered in browser

http://easylist.adblockplus.org