A "ground" in a circuit, can be a reference point from which measurements are taken. It is also a return path for current, after it passes through a load.
In mains applications, 'ground' (as daviddoria said) is something that is electrically bonded to the Earth (the soil you stand on). Eg, water pipes, earth stakes + associated wiring. Depending on what area you live in, and the local regulations, the neutral may be connected to the earth (this is called the MEN system, or Multiple Earthed Neutral)
daviddoria... Although in modern cars, the negative is connected to the chassis, many years ago, it used to be the other way - the positive was connected to the chassis. This is 'positive grounding'.