The game's rules are identical to the game show's: A question is posed, and both the single contestant and members of the "mob" must guess at the answer. If the contestant gets the question wrong, the game is over and the prize money is split up amongst the mob. If the contestant gets the answer right, members of the mob who got the question wrong are eliminated, and the value of each question goes up for the solo player as the mob thins out. The contestant can opt to quit with their winnings at any time, or press their luck and continue on.
Source: Wikipedia, "1 vs. 100 (Nintendo DS)", available under the CC-BY-SA License.