Welcome to Flyertalk.
P used to refer to the front cabin on three cabin domestic transcon flights (like JFK-LAX/SFO) but now that cabin is labeled F, not P.
P now refers to a certain type of YUP booking class; some YUP fares book into A, and some book into P. But P no longer refers to an aircraft cabin like it used to.
Now that we have that out of the way, the First Class cabin on those transcons is certainly nicer than regular domestic F. Better food, nicer seats.
Which brings us to the real question: Exactly what do you want to know?