Let me help you understand the difference by giving an example.
I filed a provisional application for patent on 14 Dec 2010 and subsequently I filed a complete specification (non provisional application) on 13 Dec 2011. The priority date will be 14 Dec 2010.
So priority date is the earliest filing date as on which the novelty and obviousness of your invention is assessed against the prior art. If you have not filed a provisional application and only filed a complete specification, then the priority date of your application willl be the date of filing of complete specification (priority date and filing date will be same in this case).
If there is a divisional application, the priority of the divisional application will be the earliest date of filing of parent application.
I hope this clarifies. Still in doubt!! please let me know.