The short version: your search results are tailored to the type of person Google thinks you are based on your search and clicking history.
Nothing really evil or mysterious if you know it's going on. But it does become a little more evil when you think about how people then start to see the web the way they want it to be seen. You'll gradually start seeing less of the things that you disagree with and/or don't like and only the things you do like and agree with - regardless of whether the thing you disagree with might in fact be the best result for your particular search.
I'm going to start by setting DuckDuckGo as the default page that opens when I open a new browser window. So I'll still use Google in the top right corner for Safari searches some of the time, but when I start fresh from a new window I'll start with DuckDuckGo.
One thing I don't like already is if I do a vanity search for my name, DuckDuckGo's results show my mostly useless LinkedIn page as the top result, where Google's results show you this website as the top result 2 - which is what it should be, in my opinion.