In limit hold 'em, bets and raises during the first two rounds of betting (pre-flop and flop) must be equal to the big blind; this amount is called the small bet. In the next two rounds of betting (turn and river), bets and raises must be equal to twice the big blind; this amount is called the big bet.
The reason most poker players prefer No Limit Hold'em to Limit is that it allows you to be more creative. In the latter, the size of your bet is always restricted, which means it's hard to make deceptive moves.