1 person is the contestant and the 100 are the "Mob." Both sets answer trivia questions--if the "1" person is right, he wins money based on the amount of people that get it wrong. So if out of the 100, 8 get it wrong, he wins $800, and it becomes 1 vs. 92. The dollar amounts increase per question and the contestant (the 1) continues to play until he decides to take the money and quit OR he misses a question. If he MISSES the question, the remaining members of the mob split the pot. So if the guy is playing and it becomes 1 vs. 27 and his answer is wrong, and the pot is 86,000, the $86k is divided by 27 and given to the remaining members of the mob.
The contestant also gets a couple of lifeline options, but I don't know those details off-hand.