What is Alexa and How It Works?

If you are a webmaster or an Internet aware person, then you might have heard about Alexa. You migh have spent few hours on searching "What is alexa", "How Alexa Works", "How Alexa Measures Traffic" etc. Well, if you are a serious webmaster then you might be aware of this fact that Alexa rankings are also very important factor which an advertiser or a web analyst looks for. So, I'm writing