Persistent Memories

What happens when a memory disappears? Once upon a time, I could actually recall the details of organic chemistry. But then I took the class final and promptly forgot every piece of information related to the chemical properties of the carbon atom. This raises the obvious question: What does it mean to forget? Do we actually delete our memories, like an unwanted computer file expunged from the hard drive? Or does the memory always remain, a persistent neuronal ensemble that we just forget...