Eli Pariser, former director of MoveOn.org, noticed that he and his friends ended up with very different search results when searching for the exact same things. Google (and other sites) are filtering out the stuff you might not like, putting you inside an isolated search filter bubble. This is a problem, but one that's easily remedied.
As Pariser explains in the video above, Google uses 57 criteria specific to you to filter your results. They include things like your location, the browser you're using, and the links you click on. Google isn't alone, as Facebook and Yahoo (among others) pay attention to this kind of information as well. If you want unfiltered results, you've got a couple of options.
The Simple Option
The easiest thing you can do is use a search engine that doesn't filter results. One of our favourites is DuckDuckGo. Aside from providing you with the same results as everyone else, it has a bunch of handy shortcuts.
The "I Still Want Google" Option
If you like Google and wanted to continue using it as your search engine, you just need to prevent it from using information about you. Activating private browsing (or "incognito mode" in Chrome) is a start, because it won't save any cookies, but that doesn't solve every problem as Google uses 57 criteria to choose the results it shows you. In addition to disabling cookies you'll need to disguise your browser agent, mask your IP, and a whole bunch more. Downloading and configuring Tor to browse anonymously might be your best bet since it's pretty tough to hide all this information.
All in all, if you're not searching for anything important it shouldn't matter much whether or not your results are filtered, but when you're looking for political opinions or accurate research you might want to use a search engine that isn't trying to show you only what it thinks you want to see.