I've seen "New York Times Bestseller" written across the top of more books than I can count. It seems every other book I look at has this advertisement, so much so that I discount it as a meaningful accomplishment.
But what does it really mean? Is it based on number of copies sold? Does the NYT actually have editors read the books and only choose ones that they enjoyed enough?
It means that a book has been - for any length of time - on the New York Time's Best Seller's List. This list aggregates sales data from thousands of venues in the United States for several different genres of books. It gathers data weekly and the data is published two weeks after data gathering (so the November 28 print edition contains the November 14th data).
Being a New York Times best seller is not necessarily an index of quality. It is an index of "hotness." A book is moving fast to make it onto that list. The idea behind advertising it is to help the book pick up even more steam. If a book spends several weeks on the list, this is an indication of a book that sells extremely well.
The reason that it seems like this encompasses so many books is that you hear about books that are NYT best sellers because they are being purchased, read, and talked about. The reality is that this list contains fewer books than you would think.
From http://www.nytimes.com/best-sellers-books/overview.html:
Rankings reflect sales, for the week ending November 7, 2010, at many thousands of venues where a wide range of general interest books are sold nationwide. These include hundreds of independent book retailers (statistically weighed to represent all such outlets); national, regional and local chains; online and multimedia entertainment retailers; university, gift, supermarket, discount department stores and newsstands.
