Limit Poker
Limit poker, also called fixed-limit poker (FL), is just what it sounds like. Bets and raises are limited to a fixed size; you cannot make a smaller or larger bet. For, example, if you play a $1-$2 Limit game, the bet sizes would be:
Preflop: $1 (AKA a small bet)
On the flop: $1
On the turn: $2 (AKA a big bet)
On the river: $2
The above table shows how much one bet would be at any stage of the hand. So before the flop, one bet would be $1. If a player bets $1 and another player wants to raise him, that player must raise exactly $1, so the final bet would be $2.
On the river, one bet would be $2. If a player bets $2 and another player wants to raise him, the player must raise exactly $2, so the final bet would be $4.
Source: pokertips.org