Alexa.com is a traffic monitoring website created in 1996 and brought out by amazon.com in 1999. Alexa.com main purpose is to monitor all the visitors viewing your website and to give you a country rank and a global rank based on your websites popularity.
Anyone can sign up for a free account at alexa.com and claim their website. Claiming your website will allow you to add a few details to give you a better experience and traffic monitoring service, you are able to add information like logo, country your website is based in, email and more.
Many users/visitors use Alexa to view a websites popularity for many reasons, one common reason is to see if for paying for advertisement on that website is worth doing or when it comes to selling a site to see if the website has a good visitor flow. Other uses that tends to happen a lot between webmasters is braging rights, as you all know that webmasters love to show of and using Alexa is a great way of doing so if you ank is better than theres.
Alexa only has one tracking method and thats via its toolbar, if your visitor has the Alexa toolbar installed then it will register the hit and if they dont have the Alexa toolbar installed they wont be counted. Once Alexa receives an average over the course of 3 months, they will multiple it by 3 or 4 and then give you a rank and slot you in the line depending of the results.