There are a number of good articles on this subject, such as What is the difference between Severity and Priority?
However the key thing to realize in the ITSM perspective is that Priority is generally a function of Impact & Urgency. Some consider Severity & Impact to have alot of commonality.
Here is a common Impact Urgency Matrix's out there:
The reasoning in many cases is to allow a end user to select urgency, and perhaps impact, but never priority directly. The reason is that allowing direct priority entry will always inflate the intended numbers (e.g. to end user, they see a p3 as a p2, a p4 as a p3, etc.)
