There are two types of ground in
electrical engineering:
earth ground and
common ground. Earth ground is "proper" ground by definition, but both types of ground are so commonly referred to as just ground that you shouldn't assume either.
Electrical Engineers are most callous, unfortunately.
Earth ground refers to the earth because ground is often found by shoving a rod into the earth. The excess energy in the circuit, be it a radio transmitter, house, or what not, is channelled to the ground, and dispersed safely into the earth.
A subtype of earth ground is chassis ground. This type of ground is also used when you ground yourself, connecting yourself to a case or other big metallic object that is not necessarily sunk into the earth, but serves the same purpose. For example, when working with logic circuits or other sensitive components in a computer, you should use a grounding strap or hold onto the case so that you don't develop a charge of your own that you transmit to the component, destroying it.
The other type of ground is common ground, or more appropriately, common. Common is the point from which a DC circuit measures zero volts. Ground was appended, and then the "common" was ditched because ground is often also used as common. Since ground is sunk into the earth, this rod will have 0v, because the earth is considered to have the baseline electrical energy and volts are measured as a difference, and naught minus naught equal double naught. However, in small circuits (like a walkman) the ground is obviously not going to be connected to the earth, and in a low power circuit, there is not much need for an earth ground anyway.
The key to a ground, be it earth or common is that it has a single, constant voltage that is used to dissipate excess energy and current in the circuit.
A little rule of thumb might help, here. In my experience, if you are referring to the ground of an AC circuit, and absolutely any electrical power system (such as a building electrical system), you are most likely referring to earth ground. In a DC circuit, it is probably common ground.