In addition to the regular tribes that you will be placed on tonight during the cast reveal, all of you will be mailed with a side group, or "profession," that you are in. There will be 5 professions, each with the same # of people. Unlike the tribes, these professions are randomly assigned and you will be associated with your profession for the duration of the premerge (unlike the regular tribes, which will fluctuate very often, as you will come to find out).
You don't really need to worry about the professions if you don't want to BUT THE IMPORTANT PART IS THAT THE TWO PROFESSION GROUPS (the names of the professions are: Gods, Mortals, Warriors, Conquerors, and Spirits. None of these are relevant to the game, they're just random names so don't read into them too much) WITH THE LOWEST AMOUNT OF PLAYERS REMAINING IN THEM BY THE FINAL 14 WILL GO TO A JOINT TRIBAL COUNCIL AND VOTE OFF TWO PLAYERS IN SUCCESSION, AFTER WHICH THE MERGE WILL OCCUR AT FINAL 12.
So, essentially, you have a vested interest in trying to keep your profession-mates around for as long as possible even if they are not on your tribe, otherwise you will have to face a joint TC at final 14 where there is a chance that you will be voted out right before the merge.
ONLY YOU AND YOUR PROFESSION-MATES WILL KNOW WHAT PROFESSION YOU ARE IN. THEY WILL NOT BE PUBLICLY REVEALED UNTIL THE PROFESSION TRIBAL COUNCIL.