Craig Naudain (SAIC) once described the difference between a System Engineer (SE) and an Operations Research and System Analyst (ORSA) as follows: an SE is the guy who tells the General he’ll go figure out an answer to the General’s problem. An ORSA is the guy who tells the General he’ll go figure out a problem to match the General’s answer.
While I love that definition (and have seen it in action), the primary distinction is reasonably well understood by comparing the concept of Development Requirement to that of Operational Requirement.