QlikView 12 uses the Qlik Sense associative engine (Qix) instead of the regular QlikView one. That's the main difference, although the effect on you as a customer is close to zero (understandably, Qlik tries to make them 100% compatible).
You shouldn't move to Qlik Sense for no good reason (or just because Qlik tells you to, they don't). QlikView will be with us for a long time, and Qlik Sense - although racing to get there - still hasn't reached feature parity.
Also, the two products are different in their capabilities and extensibility. QlikView has a lot of out-of-the-box, ready-to-use capabilities but going beyond that requires some effort, Qlik Sense is incredibly extensible, adaptible and can seemlessly integrate with almost anything on this planet. Consider Qlik Sense as your BI toolkit for any new project where mobility, device independence, integration and app-style layout and ease-of-use are important requirements. That's my take at the moment.
Best,
Peter
