Invitational means just what you think - you've got to be invited - either by winning a qualifying event or being drawn in a pool of "other qualified" teams.
The AR is the KCBS big event. There are actually two events going - the invitational and the Open - the Open is you and about 400 other teams!
Memphis in May is another big event and is very invitational (you gotta win a qualifier or get lucky in the pool drawing), same with the Jack - win a qualifier, etc. and you get into the pool for an invitation. The MiM is very pork centric - no brisket - and has a whole hog division, too.
And then there are state championships, the national cookoff in DC (nice prizes), a couple of "best of" cookoffs with merged rules and invites to the top teams of many of the "associations", etc.
It's easy to get confused. If you are going to start competing it's probably best to pick one organization for your first few sorties. As ticked off at FBA as Tim and I are it has still been a good learning experience and we were able to compete close to home.
KCBS is fine, too, if it's convenient for you. Rules are similar except for garnish in the boxes and turn-in schedule.
Go to a couple of events (like Okeechobee or Wauchula) and ask questions - just don't try and get answers after about 10 a.m. :D As you start getting closer to the turn-in times some teams get anal!! Even Tim gets "testy" as we get close to turn-in!!
Look at the rigs and ask plenty of questions - I would advise not trying to get all the answers from one team! You'll see Lang 84s, occassionally a Klose (I'm talking about FL events), many of Smokey's smokers (look a LOT like Spicewines rigs), WSMs, homemades, etc.
I'm thinking about judging at Okeechobee and Wauchula. If you make it to one of them look the Brethren hat.