This question comes up every year. The discussion follows the same path every time--i.e., someone says they are the same, someone else says that it depends on the company, and then people start talking about mystical differences.
Do a search of eng-tips.com for the terms and see if one or more of the threads answers your underlying question.
Usually one is referring to the maximum pressure the piping can handle and the other is referring to the maximum pressure the process is capable of creating. Obviously the max operating pressure should be less than the system design pressure (if 'system' is referring to the piping and not the process within the piping).
