The Myers-Briggs Personality Type Indicator was developed in the 1950's by Isabel Myer and her mother, Katherine Briggs, based in part on Jung's work on psychological types and harkens back to even earlier works concerning temperament and behavior.
In essence, there are four continuums of temperament, which when combined, give an indication of the underlying temperament of the individual. The continuums are:
No one is purely of one type. The Continuums describe behavioral or psychological modes, whether innate or developed. These modes may or may not change over time. Moreover, modes exercised in one venue may not be the same as modes exercised in another.
For the writer, the personality types can be helpful in determining how a character makes choices and reacts afterwards. For example, neither Kirk (ESTJ) or Picard (ISTJ) will be likely to dwell on past decisions unless those decisions went disastrously wrong. Neither is likely to put off a decision waiting for more information, choosing simply to make it and live with the results. They will both prefer to make their decisions based on logical and objective criteria. Both are likely to describe themselves as practical and factual, even though both are capable of acting on hunches and taking intuitive leaps that will be backed up later by logic.
Kirk appears to be an extravert, preferring to be around people. Picard appears to prefer working in small groups or alone.
When using either of these characters, for Picard to be depicted as going to a party after a long hard day and enjoying it would appear out of character for him, as would a depiction of him repeatedly putting off a decision to get more information, or worrying about a decision once it had been made. Kirk, on the other hand, will probably enjoy the party and even feel more energized being around people. This does not mean either character will make ill-considered decisions, nor will they ignore the consequences of those decisions, but neither will play the 'if I only had more information' game or dwell on past bad choices beyond the determination to do better next time. (Granted, Kirk and Picard both have had instances of obsessive behavior concerning specific incidents, but these are always presented as anomalous behavior.)
Some books on the subject: Personality Typing & Jung
Other sites: Personality Tests and Types