What is Pagerank ? What is Alexa rank? What is Backlink ?

What is Pagerank?
PageRank is a numeric value that represents how important a page is on the web. PageRank is an index used by Google to value Websites on a scale of 0 to 10, based largely on link popularity. Pages that google believe are important pages receive a higher PageRank and are more likely to appear at the top of the search results.
What is Alexa rank?
Alexa.com is a subsidiary of Amazon.com and is a website which provides information on traffic levels for websites. The traffic rank is based on historical traffic data from millions of Alexa Toolbar users and data obtained from other, diverse traffic data sources, and is a combined measure of page views and users (reach). The Alexa Certified Traffic Rank shows your users the Traffic Ranking number for your site. The lower the Alexa ranking number the more heavily visited the site.
What is Backlink?
Backlinks are incoming links to a website or web page.