How is a side pot is shared?

A. Say three players are in the hand: Player 1-3.

Going to the flop there is $100 in the pot and the three players left have these totals:

Player 1: $100
Player 2: $125
Player 3: $50

$50 is bet on the flop. $250 is now in the pot, but now only Player 1 and Player 2 have money ($50 and $75 respectively). Any money they bet from this point forth will go into a side pot. Say they bet the remaining $50 Player 1 has.

Main Pot (for Players 1-3): $250
Side Pot (for Players 1&2): $100

First, you determine who has the best hand between Players 1 and 2. Player 1 does -- so he gets the $100.

With Player 2 eliminated, Player 1 now competes against Player 3 for the main pot. Here Player 3 has the best hand, so he gets the $250 only (Player 1 keeps the $100 from the side pot).