It's a pickup game of Arena PvP. Everybody joins the match, 2 captains are selected, and they take turns picking players. It's generally open to anyone who wants to join, but be thick-skinned as you will be the fat kid that gets picked last at first. It's also a step up in competitiveness from zones, so bring a reasonable build - it doesn't have to be super purpled and pvp io'd, but have SS/SJ, 41 kb protection, lots o' hit points, etc. Also, have Ventrillo installed before showing up, and ask for Vent settings when joining the match.
Casual Arena Match where two captains are either picked or volunteer. Captains then take turns picking members for their team from the remaining pool of players until all are picked or all 8 slots are filled.
If there is an odd number or more than 16 players the remaining ones are the fat kids, where basically they don't get to play or have to sit out for that round.
*Edit* Damn you Pixel. Last time I try to be helpful.
O snap, Lib is late on the spike, again.
Infinity and Victory mostly
dUmb, etc.
lolz PvP anymore, Market PvP for fun and profit
You know you're good when you are calling targets, aren't even locked yourself and struggle to make your own countdown on a few spikes. 
Ah ok! So just a slightly more organized version of the old Arena matches when they first came out. (Or so it sounds) Thanks for the answer! That was much easier than I thought. 
...I think. I'm going to just bite the bullet and ask this question since I haven't found out the answer by now. What is a kickball game
I usually lurk til I find something out that everyone seems to know but not seen anyone else ask or read an explanation elsewhere. /noob