1. It is a derivative of the intransitive verb party.
2. It is the phonetic spelling how some "people" have decided to pronounce party.
Has nothing to do with determining the attitude/type of the party. It just so happens a selective group of people(?) use this word(?) and hence, the style of the parties held are incidentally similar (e.g. usually containing loud rap or hip-hop music, an abundance of substance abuse and often a great amount of sexual promiscuity. Source: bud newman., Feb 28, 2003).
The infamous line:
This is pronounced with a definate number of exclamation marks depending on:
a) how excited you are and
b) how high you are.