Each letter in the personality type code - I, S, T, and J - describes a preference for a way of thinking or behaving. There are eight styles and you use all of them, but ISTJs prefer:
Introversion (thinking things through) more than Extraversion (interacting with people)
Sensing (perceiving tangible facts) more than iNtuition (perceiving new possibilities)
Thinking (making decisions using objective logic) more than Feeling (making decisions using subjective values)
Judgement (an organised lifestyle) more than Perception (a flexible lifestyle)
If your closest personality type is ISTJ then you are interested in clarity and knowledge. You like to observe and listen, and have a particular interest in facts and information which help you to develop as clear a knowledge as possible. You like to know where you stand, e.g.: having clear goals to which you are working, and to know that what you are expected to do is achievable. You value your experience, which serves as a strong guide to your decisions.
Stereotypes and Individuality
The original author of personality type theory - Carl Gustav Jung - said that everyone is individual and unique. The personality types are not strict classifications, but stereotypes that are akin to landmarks on a map. Just as a few landmarks can help you find many unique locations, so too the personality stereotypes can help you understand your unique personality.