If the last web site you visited was for the Audubon Society and the next thing you search for is Cardinals you will get a page about birds on the first result, but if you had just recently looked for tickets to a ball game on Ticketmaster you will get the baseball team as your first result.
One of the more fascinating things that DDG's "Dontbubbleme" campaign did, was expose just how big a swing in the results your 'meta data' has. People who are running Chrome and are "logged into Google" get very different results than people who make a query with no context following them around.
In the latter case everyone has to resort to giving you the pages that "most" people clicked on from your geographic region [1].
Bing and Google have pretty much exact equivalence if you remove tracking and geo context. (IP blind / cookie blind / search).
[1] For even more fun, set up proxy servers in various data centers around the country and do searches with no cookies or context, and proxied to different geographies. Very enlightening.