What is Alexa Website Ranking?

Alexa is a search engine where the results are based on different rank for the websites around the world!!In theory alexa calculate this rank on different basis such as how much visits,popularity,links in popular sites as social bookmark,featured contents and so on...So if this is really true as results i don't know but generally you can see listed all sites which they have great popularity worldwide even if sometimes the crawler of alexa cannot transfer all dates for every website and so some sites are listed with no data but this not mean that site had not visitors but just the difficulty to keep dates from the alexa crawler as well!!As regards the amazon is simple alexa engine is owned by amazon company and so no further links as regards the principal function of amazon such as ecommerce!!