The branch of philosophy that deals with the nature of knowledge and our means of gaining it.

Epistemology deals with how we gain knowledge. It describes how we gain information about the world, such as through our senses. It describes the nature of concepts, and how they retain objective information about the world. It describes logic, and how it can be used to identify and resolve contradictions.

Epistemology isn't concerned primarily with what the nature of the world is, but with how we gain knowledge about it. The nature of reality is described in the branch of philosophy called Metaphysics. Epistemology and Metaphysics aren't unrelated, though. What you view as the nature of the universe inevitably impacts your theory or knowledge.