There are two types of the open the box game.
The first is the game with lots of keys and you get to pick a key.
Then there is the one with questions and multiple boxes
Pick a Key
In this game you get a raffle ticket, or tickets for the chance to open the box for a cash, or other prize.
If your ticket is picked you get to come up and pick one of upto 10 keys and try and open the box. If you are successful then you win the prize, if not the money from that draw is added to the prize and play continues until there is a winner.
Multiple Boxes
With this game you answer a simple general knowledge question and if you get this correct you get the chance to open one of several boxes or take the envelope.
If you take the envelope you win what is in it (usually a free attempt to try again or a free drink etc.). If you open the box you win whatever is in the box.
Now the prizes in the box could be money, lottery tickets or nothing at all, the choice is yours.