Basically, Alexa measures site popularity/traffic. It does this through info fed to them from people who have installed the Alexa toolbar. So it's not a true and accurate measure of the most visited sites on the Net. It's a reflection of the most visited sites by people who have the Alexa Bar installed.
The lower the number the better. For instance, #1 is Yahoo.com. WHT is under 1000, making it a very popular/visited site for Alexa Bar users.
Keep in mind the demographics of Alexa Bar users. Generally, IMO, people who surf the Net occasionally aren't savvy enough or won't even know about Alexa. So sites that will do better in Alexa rankings are, IMO, ones whose target market is more "experienced" web users.
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