How can anything be more than 13.7 billion lightyears away?

I have very little knowledge about the astrophysics and the universe, but I HAVE been aware that our universe is known to be approximately 13.7 billion years old. Now, knowing this, I would automatically assume that the furthest objects from earth would be no more than 13.7 billion lightyears away. I recently found out that there is more than just the "observable" universe, and that scientists are talking about 78 billion light years! Could someone please explain this to me. Also, it would help if you explained it the simplest way possible... :)