hi,
Alexa is a Web Information Company, they are rank website on many factors like traffic. Alexa rank is updated daily.
How Alexa Works?

Alexa.com was found in 1996 and they collected data for ranking website through a toolbar.

Many people think (Including Me) Alexa does not give an exact ranking for websites.

But,

They give the best metric for comparing two sites
Thanks.
sonvi.belani

Alexa shows the crawler and human traffic based ranking and location based ranking.

Alexa’s traffic estimates and ranks are based on the browsing behavior of people in our global data panel which is a sample of all internet users.

Alexa is a tool helps you to estimate the power of a website with traffic.

Alexa Internet, Inc. is a California-based company that provides commercial web traffic data and analytics. It is a wholly owned subsidiary of Amazon.com. Founded as an independent company in 1996, Alexa was acquired by Amazon in 1999.

Alexa is one of the oldest and most popular traffic ranking tools widely used by webmasters, bloggers and Internet Marketers. Alexa Rank is the rank of online traffic to a particular website, in comparison to all other sites on the web. The rating is provided by Alexa.com, The ranking is made up of data compiled only through those who have the toolbar. Therefore, the Alexa rank is not a necessarily a reliable metric. There may be a lot of traffic that is not being taken into consideration.

Alexa rank sites based primirily on tracking a sample set of internet traffic users of it toolbar for the internet explorer,Firefox,Google chrome webmasters.,