Humans and robots

Those are the two categories of characters (aside from "animatronic"-like beasts). The latter, robots, known euphemistically as "hosts", are relatively similar in status, though as the series progresses it's evident that there's a hierarchy amongst them.

The humans are more complicated. The paying customers, called "guests", are the simplest and most oblivious, presented, with a few exceptions, as morally evil. Then there are the technicians, also relatively simple, and presented as mostly amoral (again, with exceptions). Then come the mid-level managers, who are given various possible agendas, and are presented as largely cynical and ambitious. And then the high level managers, with direct links to Corporate;  the owners themselves; and finally, at the highest level, the surviving creator/designer, Ford. There are a few glimpses of people outside the world and business of the Park, but they have little or no role.

The most striking thing about this character dichotomy on the whole, however, is how it upends the conventional moral relationship of human and robot. At best, "mechanical humans" have usually been presented as "robotic" imitators, comical or threatening as the case may be. But here, at the start the viewer is deliberately deceived about the robot Teddy, presenting him as simply one of the arriving "guests". And then we're soon shown situations in which it's the robots who come across as archetypically human, and the humans who seem morally dubious, and then vile. This reversal, sharply divergent not just from the movie that gave the series its name but from virtually the entire genre of the artificial human, lies at the heart of the project as a whole.