what is a gambit?

what is a gambit? more specifically the queens gambit.

Where you sacrifice something (usually a pawn) to gain an advantage in developement, control, or an attack.

The Queen's Gambit is when you play 1. d4 d5 2. c4

This sacrifices a pawn which will probably be captured later by your bishop.

Also, gambits can be accepted or declined, which means the other player either accepts your sacrifice or continues normally.

I dont know that much about gambits but i can tell you what i think i know...
A gambit is where a player sacrifices a pawn in order to gain an advantage of some sort. One result: a gambit is used in order to gain positional advantage where your pieces are now in comfortable positions and can proceed to induce pressure on the enemy position. Another result: when a gambit is accepted... sometimes it becomes difficult to develop efficiently. Often gambits are a mix of these two factors.

Now for the second part of your question... the Queens Gambit is a particular opening that is moulded around a gambit that white offers in the second move:

A gambit is moving any piece into a square where it can be readily captured by the opponent, usually to gain some sort of advantage in position, with the intent of equalizing (or gaining) material later on.

Mostly, it's a sacrifice.  But many times it's not good to accept gambits.

Gambits are more specifically opening sacs (usually pawns) for some positional advantage.  after the opening, other sacrifices are called sacrifices :)

I think gambits are useless. I've won against every single gambit player; either king or queen, either white or black. Their sacrifices are completely useless.

Gambits are fun but risky.
Most gambits are unsound: the material deficit remains while the compensation evaporates.
Good are the Queen's Gambit, the Catalan, the Two Knights Defense, the Marshall Attack.

There are no useless openings, only useless players.

A gambit in chess refers to a strategic sacrifice of material, usually a pawn, with the hope of achieving some advantage such as rapid development, control of the center, or an attack against the opponent's king. The Queen's Gambit is a specific opening gambit that begins with the moves:

1. d4 d5
2. c4