When I tell people I have a masters degree in Operations Research they often say "What is Operations Research?". Michael Trick has a great blog post examining this very question of explaining Operations Research to others. In fact he even solves that question with the ever so appropriate Operations Research tool of using an algorithm.
My typical response to the question "What is Operations Research" is using mathematics and statistics to help solve a business decision. Better yet, from the blog article, is Jim Orlin's response. Jim states Operations Research is "science of better decision making". Very good response and is one I will use in the future.