A use case is a sequence of interactions that a user makes with a product to achieve a goal. When your user interaction designer fleshes out use cases, she has a wide range of choices of how specifically she describes the interactions.
If she chooses abstract language that is relatively free of technology and design assumptions and user interface details, then the use case is essential.
If, on the other hand, she specifies concrete steps that assume technology and design commitments and details about the user interface, the use case is real.
Essential and real use cases are on the extreme points of a continuum. No non-trival use case is purely essential (often, the very order of steps in a use case represents a design decision), and no level of detail in a use case can render it purely real (there is always some room for developers to make additional technology choices).