1. Google search - google search engine 2. content network - adsense related sites make up the content network. 3. search network - google syndicated search partners like aol (they power their search engine using google)
There are some large sites that carry the AdSense programme, myspace, youtube and New York Times etc. Placement targeting allows you to choose the websites your advert appears on down to individual URL's Hope this helps
Your advertising on webmaster sites displaying adsense. I don't think it does because it's owned by google.