What is Service Design?

So what exactly is service design? Change management, design consultancy, interactions design, operations management?? 

Here are a few quotes I came across whilst trying to find an answer to the same question...

"Service design is all about making the service you deliver useful, usable, efficient, effective and desirable"   -  UK Design Council

"When you have two coffee shops right next to each other, and each sells the exact same coffee at the exact same price, service design is what makes you walk into one and not the other"   -  31 Volts Service Design

Great. But if a service is not a product or even something you can keep, and therefore not really tangible, how do you go about designing one and how do you know if it is any good?  How do you make sure that your coffee shop is the one that people "walk into"?

Over the next 3 months I will be studying service design and working on a project simultaneously. Hopefully when my project is complete I will be more than able to answer these questions - no pressure then.