A theory is a model of reality that helps us to understand, explain, predict, and control that reality. In the study of personality, these models are usually verbal. Every now and then, someone comes up with a graphic model, with symbolic illustrations, or a mathematical model, or even a computer model. But words are the basic form.
Different approaches focus on different aspects of theory. Humanists and Existentialists tend to focus on the understanding part. They believe that much of what we are is way too complex and embedded in history and culture to "predict and control." Besides, they suggest, predicting and controlling people is, to a considerable extent, unethical. Behaviorists and Freudians, on the other hand, prefer to discuss prediction and control. If an idea is useful, if it works, go with it! Understanding, to them, is secondary.
Another definition says that a theory is a guide to action: We figure that the future will be something like the past. We figure that certain sequences and patterns of events that have occurred frequently before are likely to occur again. So we look to the first events of a sequence, or the most vivid parts of a pattern, to serve as our landmarks and warning signals. A theory is a little like a map: It isn't the same as the countryside it describes; it certainly doesn't give you every detail; it may not even be terribly accurate. But it does provide a guide to action -- and gives us something to correct when it fails.
Usually when we talk about someone's personality, we are talking about what makes that person different from other people, perhaps even unique. This aspect of personality is calledindividual differences. For some theories, it is the central issue. These theories often spend considerable attention on things like types and traits and tests with which we can categorize or compare people: Some people are neurotic, others are not; some people are more introverted, others more extroverted; and so on.
However, personality theorists are just as interested in the commonalities among people. What, for example, does the neurotic person and the healthy person have in common? Or what is the common structure in people that expresses itself as introversion in some and extroversion in others?
If you place people on some dimension -- such as healthy-neurotic or introversion-extroversion -- you are saying that the dimension is something everyone can be placed on. Whether they are neurotic or not, all people have a capacity for health and ill-health; and whether introverted or extroverted, all are "verted" one way or the other.
Another way of saying this is that personality theorists are interested in the structure of the individual, the psychological structure in particular. How are people "put together;" how do they "work;" how do they "fall apart."
Some theorists go a step further and say they are looking for the essence of being a person. Or they say they are looking for what it means to be an individual human being. The field of personality psychology stretches from a fairly simple empirical search for differences between people to a rather philosophical search for the meaning of life!
Perhaps it is just pride, but personality psychologists like to think of their field as a sort of umbrella for all the rest of psychology. We are, after all, concerned about genetics and physiology, about learning and development, about social interaction and culture, about pathology and therapy. All these things come together in the individual.