I'm interested in the study of Personality Types.
That's like phrenology, right?
How is this different from, say, horoscopes...positing peoples' personalities based on the sign of the zodiac under which they were born?
This is very different. With a horoscope (or phrenology :-), I would take one fact about you and try to extrapolate knowledge of your personality, ideal career, likes and dislikes, or your future, from that one fact.
With MBTI or Keirsey, you describe your own personality, based on many factors. You answer a series of questions, identifying your likes and dislikes, your interests, what sorts of activities energize you and what things leave you cold. You identify your personality type.