A gambit is the giving up of a pawn in the opening to gain a lead in development and/or open lines and/or a weakening in the opponent's position.
A sacrifice is the giving up of a piece to gain what a gambit does. It also can be used to close lines, setup a blockade, attract a piece to a square, destroy the pawn shelter around the king, and other gains.
Since a sacrifice is more of an investment, it usually involves an attack on the king.
The Queen's Gambit is normally not a true gambit as White can always regain the pawn. The Queen's Gambit starts with 1. d4 d5 2. c4. The pawn is offered to lessen Black's control of the center. This action is considered good according to chess strategy.
Sooner or later you will have to face a gambit. Learning before the game greatly increases you chances of not losing in the first ten moves. Games in which a gambit is accepted are often full of tactics and a slashing king hunt. These games are considered to be more exciting.