What is Google page rank?
Page rank is a method where Google rates an indexed website on a scale of 0 - 10. The criteria for ranking goes like this. Firstly it depends on your popularity. The higher the visitor rate, greater is the ranking.
In addition these contacts must be popular ones. The more than number of popular people, greater will be your rank. The popularity of these popular people rubs off on you.
Authority websites that vote for you also matters. Websites that end with .gov, .edu are all authoritative and bear more value. Further the number of visitors to your website may be not unique, meaning they visit other sites as well. In such a case the weightage of their votes will be distributed.
Trading your links with others will cancel out both the votes and reduce your ranking. Only the best and most visited websites get good rankings.