Difference between an entity class and an entity instance?

"An entity is a person, place, event, or thing about which data is collected...An instance is an occurence of an entity." - Systems Analysis & Design (4th) - Dennis

An example of this would be STUDENT as the entity while JACK SMITH is an instance of that entity.

If an entity is an individual "person, place, event, or thing about which data is collected", then an entity is an instance. Linguistically, entity is just another word for a single thing ("The existence of a thing as contrasted with its attributes.", Merriam-Webster's On-line Dictionary). Entities that have the same attributes are grouped in what are best called entity classes (it doesn't make sense to refer to a collection of entities as an entity). Entity and entity class are data modeling terms. The corresponding object modeling terms are object and class, albeit that a class typically has operations, which are foreign to an entity class.