The simplest kind of poker tournament takes place at just one table.
Usually 8-10 players enter the event, though some sites have special
2-seat tables for heads-up play. As players "bust out" (lose their last
chip), nobody takes their place and the table gets shorter and shorter
handed. Eventually, one player wins all the chips and the tournament is
over. At that point, the prize pool is divided among the players
according to some payout schedule. Typically, the first place player gets
about half the money while second and third get smaller shares, perhaps
30% and 20%.
The "multi-table" tournament format allows a much larger number of people
to play in the same event. Online events often have thousands of
entrants. Each player is assigned to start play at a certain table. As
players throughout the tournament bust out, the poker server moves
players into the vacated seats and reduces the number of tables.
Eventually, play comes down to two opponents, just like in a single-table
event, and ends when one player wins all the chips. At that point, the
prize pool is divided among the top-finishing players. Again, first place
pays the most, but multi-table events generally pay a larger number of
players than a single-table event. Typically, the top 5% to 10% of
finishers receive some portion of the prize pool.