Alexa Internet Inc provides alexa rankings for websites based on the data collected from it's toolbars. Since the data is collected from a sample subset of the population that has the toolbar installed, it may not be very accurate. The various metrics are given below
Alexa Rank: This is the current rank that the domain has based on the last three months of data
Top Rank: This is the highest rank that the domain has ever reached
Rank Delta: This is the change in rank from the previous rank (from three months ago). A negative number means that the rank has increased from before.
Country Rank: If the domain is popular in a particular country then Alexa gives the ranking for the domain in that country
Back Links: The total back links to the domain based on Alexa data
Address Information: The registration address and details
Related Sites: List of sites related to the domain