Alexa, simply put, is a top list or ranking of the entire web; at least that is their core competency.  Alexa tracks and collects data from its vast number of Alexa toolbar users as they surf the web.  Alexa then statistically ranks every single site from 1 to however many sites are in their database (millions).  The number one site is the one that gets the most hits, currently Yahoo, followed by number 2, Google, and so on. Think of Alexa as the Neilson television ratings, only for the internet.

Ok great, so how is Alexa useful for optimizing your site?  While you can't use Alexa  to generate traffic, you can use it to keep an eye on where you rank and where your competitors and potential and existing partners rank.  Useful features of Alexa statistics include: