Alexa is a Web information company
On Alexa.com You can find background information about websites. Alexa started in 1996 with a toolbar that you could install in your InternetExplorer browser and that kept the sites you visited. Based on that data, a website was given a ranking (AlexaRanking). This principle has not changed, but since 1999, Alexa has become a part of Amazon. Data from Alexa are often used to see how popular a site is, but the data is not really reliable: indeed, Alexa only has data from the people who installed the toolbar. There are millions of people who have done this, but it is still a sample of a number of people on which the data are based.