Google search results have always been based on relevancy, and the method for tracking that relevancy has always been evolving. Originally the measure of relevancy involved a formula that considered the keywords you used in a search query and the PageRank, which measured the backlinks leading to a particular page. The idea was that websites with lots of links were more important and therefore more relevant to most searches. That's not a bad way of looking at search results, but it's a system that can be gamed, and it's been heavily gamed over the years. Even JC Penney got in trouble for the practice when the New York Times exposed what appeared to be a blackhat linking practice.
So PageRank alone is out. Another way to measure relevancy is to make results highly personal. Throw in results that are geographically near the person searching. If you're looking for a haircut, you probably want the local salon and not just the most popular results on the Web. Also consider search history. If someone has repeatedly searched for the same thing, they're probably trying to revisit a site. If someone has been searching for Indonesia in the past, they're looking for the Java island and not the programming language.
So how else do you measure relevancy? One way to do it is to consider social metrics. It's not just a matter of whether or not lots of websites link to a site. Do your friends like it? What about the friends of your friends? Using this social data, you probably will find results that are a little more relevant. After all, if you're looking for a place to get your car fixed or a recipe for hot chocolate, aren't you more likely to ask your friends first?
Unsurprisingly, this shift has raised a lot of privacy and anti-competitive concerns.
Google addressed privacy concerns by making sure all search results for a logged-in user are on an SSL page, and Google is making some efforts to clearly label the personal results and identifying who suggested it and whether it was a public or limited post.
The anti-competitive concerns center around the fact that Google only lists Google+ results in their personal results, not those of Facebook, Twitter, MySpace or other competing social networks. That leads to the complaint that Google is essentially just launching a Google+ advertising service. However, it's not quite as simple as that. Not all of those sites are open, and not all of the use real names, which is one of the items Google has decided should be a priority in social networking services.
Engineers from competing services have tried launching a proof of concept that other social metrics could be used in Google Search plus Your World to rebut the idea that listing other services is beyond Google's current capacity. Search plus Your World is still a work in progress, and they may very well bow to pressure to expand their personal recommendations.
Removing Personal Results
If you don't want to see personal results, whether it's because the idea bothers you or because they're less relevant than your normal results, it's an easy fix. You can just log out of Google, but that's not necessary. There's a toggle on the upper right corner of your screen with a person and a globe. When you don't want personal results, just toggle the globe button, and you're done. It's sometimes instructive just to toggle between the two results to see what the difference is.