A site sponsor: These people put an advertisement on their website, and pay the owner of the website for the advertisement space. Sites like google AdWords (Words.google.com">https://adWords.google.com).
A site owner: These people are similar to the site webmaster, but have bought the domain (that's the www.sitename.com) and are in charge of the contents of the site.
A site webmaster: These people update the website, and put the content on the web that you see. These can sometimes be the owner aswell, but are not usually the site sponsor.
