This question comes up often and here's the best explanation I have found to date
Not everyone knows what quote marks mean in a search entry
From The New York Times, February 12, 2006, article by Randall Stross on amazon:
"Something in Amazon's secretive investor relations office has wafted through the air into its customer service department. The company has long provided a toll-free number for customers who want to speak with a human representative to solve a problem. Yet it does not mention the number on any help page on its site. If this guess-our-phone-number game is intended to curtail calls and to keep associated costs low, it seems unlikely to thwart anyone. A search for "Amazon 800 number" takes Google's servers an eighth of a second to produce 3.3 million results.
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Putting exact phrase quotation marks into this search yields a grand total of 760 results for the phrase, a number which differs from 3.3 million. But the whole idea in the last paragraph is wrong, since the number of results has little to do with thwarting a search; for in this case there is no practical difference in ease of finding the amazon 800 number between hundreds and billions of pages returned.
No doubt everyone here knows that searching AMAZON 800 NUMBER produces all those pages containing the words AMAZON and 800 and NUMBER. Searching "AMAZON 800 NUMBER" returns all those pages that contain the exact phrase AMAZON 800 NUMBER.
I once was talking to web designer A who said that he was more famous than some other web designer B. Designer A claimed some vast number of results in Google, far more than the wretched B. The number seemed large, so I asked A if he put quote marks around his name when ego-surfing. "No, that makes B higher than me." I almost blurted out something about designer arithmetic, but instead bit my tongue.
What is the biggest possible difference in results for words in quotes and not in quotes? Here's a guess. The exact phrase "the and" yields 0 documents. Without quotes, the phrase yields 8,200,000,000. This will hold at least until Google spiders this thread.
--Edward Tufte, February 11, 2006