I am sure no one is murmuring here around it with this : Matt Cutts the head Google webspam team tells the difference between Popularity and Authority, and it signals that soon some update may be seen. Read the full inside story from here and share your views: http://www.webpronews.com/matt-cutts-talks-about-coming-google-algorithm-changes-2014-04
as per my thought difference between popularity and authority is that if you have high page rank site that does't means that site have lots of traffic. like matt given example of porn site and gov site, i don't think so porn site have high page rank but they have large traffic. and gov site have good page rank but it doesn't have good traffic!!!
That's the thing - lots of traffic means your site is popular, but not necessarily authoritative. But if you twist Matt's word a little and you are actually searching for porn of some sorts, then the said porn site can be both popular AND authoritative (given that it has many related links from other porn websites). I don't think there is much room for confusion really, if you understand the difference between Popularity and Authority in the first place, the same applies for websites.