"Entities are a state of mind. No two people agree on what the real world view is."
An information system (e.g., database) is a model of a small, finite subset of the real world. (More or less—we'll come back to that later.) We expect certain correspondences between constructs inside the information system and in the real world. We expect to have one record in the employee file for each person employed by the company. If an employee works in a certain department, we expect to find that department's number in that employee's record.
So, one of the first concepts we have is a correspondence between things inside the information system and things in the real world. Ideally, this would be a one-to-one correspondence, ...