True censorship is done through a series of things, like IP banning or simply whitelisting. Whitelisted sites are those that are OK’d. In otherwords EVERYTHING is blocked and then one by one a site is submitted and approved. If OK’d (is that a word?) then it’s allowed. These sites undergo periodic review to make sure their content isn’t changed after they are OK’d (I guess it’s a word now:) )
Proxy servers can’t get around a whitelist. Some nations ban all proxy servers, so even if your uses is legit, you can’t use it. Some nations ban all sites from that have a certain countries web address (such as Israel’s .il (top level domain). Others will allow the .il domain so long as the IP doesn’t go back to Israel. Of course their are ways around that. Other nations like Iran make the ISP responsible for making sure content doesn’t slip through.
Mirrors can get around blocks. For instance I have a site, well call it “Markxxx.Com” (not real) it mirrors to JoeBlow.Com on a different webhost. When I update one site the other site is automatically updated. But if Markxxx.Com is blacklisted, my JoeBlow.Com site probably won’t be. 'Cause people won’t know about it, unless I tell them.
There is “effective censorship” because most people never look beyond the second page of Google results. Google is an “effective” monopoly. (This means they function like a monopoly but aren’t because they field is open, it’s just hard to break into. Another example would be eBay.)
Google bases it’s results on links. Links count differently. For instance a link FROM a top ranked site, like USAToday can be worth 10,000 points. While 1,000,000 million links from sites not ranked or bottom ranked, may be only worth 100 points total.
So it’s not only the number of links, it’s the QUALITY of those links as well. Also it’s how the links are structured. If USAToday says “no follow” in their links, any links from USA Today won’t count in Google, though they of course, still link the readers of USA Today to that site.
So you see a powerful site like USA today that’s top ranked, can control how other links are viewed. And they though use of the scripting of their webpages can control things.
SEO (Search Engine Optimization) is used to get your site to the top of the searches but can be abused and spammed. And Google often doesn’t follow it’s own stated rules. For instance, Google is supposed to filter duplicate content out, but clearly as one can see from all the sites that scrape Wikipedia, this is false. I can look up a term and come up with 7 sites in the top 10 that are scraping the Wikipedia and providing the same info.
I don’t think that is as much of a matter of censorship as fairness.
For great reading on SEO try Search Engine Watch (dot) Com