When computer vision system interpret a scene, they benefit greatly when they have knowledge about the kinds of objects they “see.” For example, medical diagnostic system need to know the characteristics of the diseases being diagnosed. With our knowledge of the application environment, or domain knowledge and knowledge of the intended audience, natural language understanding system could not properly comprehend the phrases and sentences they process. What we need ,then,is a suitable way to represent such knowledge to the systems. For this reason, artificial intelligence researchers are preoccupied with what is known as knowledge representation. It is basically the "glue" that bind much of ai together, but it has its own set of problems