If you use something like adsense, how are the ads to be placed on your website chosen? You don't get a choice do you?
In Google adsense only relevant ads are publishes. You can restrict the ads which you do not want to publish on your site. Google Adsense is flexible
Its based on content that is on your site, so if you have an ecommerce site selling dvd, adsense will display dvd related ad's If you go to a page on the site that sells plasma tv's adsense will display tv related ad's and so forth.
It depends on what content you have on your site. For eg. If your site is about movies... you will see ads about movies and stuff.
It has to do with the text on your Website. So no, you do not get to pick the ads. If you want to pick your own ads, you should try a place like Adbrite.
No, you can't choose which ads will appear on your site. But you can block some of the sites using the competitive filter inside your AdSense control panel.
adsense is advertising on content network. therefore, the ads show on your site are related to the content of your site. you just place adsense code on your site and the ads get displayed which are related to your site