In an attempt to increase the relevance of search results as well as overall usage, Google has made features such as Personalized Search, Search History and Personalized Homepage automatically available for those who sign up for new Google accounts, according to Search Engine Land.
Users will have the option to turn the features, which current users still have to manually activate, off either as a whole or individually to respect the privacy concerns of some people. But leaving it on, especially within Personalized Search, can help return better results.

Over time, the system learns what specific sites or sorts of sites you click most often and begins to bring those higher in the results when appropriate.

As the article states, this adds an exponential number of opportunities for search marketers to get their messages across because of the now almost infinite permutations of search results. Two people who run the same search will get different results because of their different search histories.