The way I understand the difference between J & P is that it has to do with the way you prefer to structure your environment, or at least, the way you do that can help you decide if you're a J or a P.
This information is taken from the Barron & Tieger website - personalitytype.com.
They ask:
What kind of environment makes you the most comfortable?
Judgers prefer a structured, ordered, and fairly predictable environment, where they can make decisions and have things settled.
Perceivers prefer to experience as much of the world as possible, so they like to keep their options open and are most comfortable adapting. So,
Judgers tend to be organized and productive while
Perceivers tend to be flexible, curious, and nonconforming.
Judgers often:
* Make most decisions pretty easily
* Are serious & conventional
* Pay attention to time & are prompt
* Prefer to finish projects
* Work first, play later
* Want things decided
* See the need for most rules
* Like to make & stick with plans
* Find comfort in schedules
Perceivers often:
* May have difficulty making decisions
* Are playful & unconventional
* Are less aware of time & run late
* Prefer to start projects
* Play first, work later
* Want to keep their options open
* Question the need for many rules
* Like to keep plans flexible
* Want the freedom to be spontaneous