For what i saw on reddit and youtube (based on bahamut raid, the first raid in JP)
You fight the boss and everybody is defeated you gain points for total damage dealt to it.
With that points you get ranked, just like arena, and at the end of the event, you get rewards for the ranking you ended (again, just like arena)
In the bahamut raid the rewards were:
Rank 1-1000 = 2 big trust moogles + 5 tickets
Rank 1001-10000 = 1 big trust moogle + 4 tickets
Rank 10001-30000 = 1 small trust moogle + 4 tickets
Rank 30001-50000 = 1 small trust moogle + 3 tickets
Rank 50001-100000 = 2 tickets
Rank 100001~ = 1 ticket
Also, there are individual rewards you got for achieving certain point milestones (like star quartz, trust moogles, friend points, stat pots)
And finally you gain rewards (tickets) for total damage the server has done to the boss.
Here you can see the rewards
It looks like raids are very rewarding events.