How to correctly know the rank and traffic of any webpage?
Google PageRank Checker
Do you mean web page or web site?
There are various tools around the internet that will give a figure but they are of questionable reliability, at best "indicative" at worst quite simoply wrong. The most reliable statistics are only available to a website owner through their log files or Google Analytics. Apart from try semrush.com, at least it gives some figures for some sites Alexa seems not to know about.
Alexa is great when it comes to traffic from "large" countries but if you site mainly get traffic from small countries Alexa do not count it right. You can see it if you compare it with Google analytics figures - they are not public
That's not possible. The public PR rank is updated rarely and accurate traffics are not public. You can use Alexa (http://www.alexa.com/) to get estimates of traffic and popularity.