How do you differentiate good customer service from superior customer service?

Good customer service is about meeting the needs of the Customer. Customers have an inherent expectation that they will be treated well, i.e. in a friendly, kind, and respectful manner. Ansering their questions and being knowledgeable about the product or service is also a characteristic of good customer service. If you deliver the product or service you promise, in the manner in which you promise, and help the customer with any difficulties or challenges they may have with the product or service, and the customer is ultimately happy with the experience, that's good customer service.
Good customer service is also about ensuring the customer is receiving the appropriate product or service they truly need and NOT selling them more product or service than they need.
Superior customer service includes all of the above as well as exceeding the expectations or needs of the customer. It's about going the extra mile to please the Customer. For example, at a recent hotel stay, I was greeted at the front desk with a gift bag which included some free snacks and drinks from the hotel. It was unexpected and made my day.
If you want an excellent example of superior customer service, I recommend you try to get a copy of the Pike's Place Fish Market video. It's a bit pricey, however you might be able to find it on the web somewhere. It also comes in book form, but the book isn't quite as "impactful."
http://www.crmlearning.com/fish-video
There's also another excellent video on superior customer service called "Give Em the Pickle."
http://www.giveemthepickle.com/give_em_the_pickle_video.htm