Different types of grounding (differences between chassis ground, earth ground and signal ground)

Ground

Ground is shown on electronics diagrams, but it isn't really a component. It is simply the node which has been assigned a voltage of zero. It is represented by one of the symbols below. Technically, any single node can be assigned as ground, and other voltages are measured relative to it. However, the convention is to only assign it in one of two ways, related to the type of power supply. In a single supply situation, such as a circuit powered by a single battery, the ground point is usually defined as the more negative of the power source's terminals. This makes all voltages in the circuit positive with respect to ground (usually), and is a common convention. For a split-supply device, such as a circuit driven by a center-tapped transformer, usually the center voltage is defined as ground, and there are equal and roughly symmetrical positive and negative voltages in the circuit.

Signal Ground		Chassis Ground		Earth Ground

Signal ground 

Ground for a signal. Since wires have a certain amount of resistance to them, ground points in a circuit aren't all at exactly the same voltage. It is important in practical circuit design to separate the power supply ground from the signal ground from the shielding ground, etc. In circuits where minimum noise is especially important, power regulator circuitry should have thick wires or traces connecting the grounds, in a sequence from the power supply to the "cleanest" ground at the output of the filters of the power supply, which will then be a "star point" for the grounds of the signal circuitry.

Chassis ground 

A direct connection to the chassis of the device. This is used for EMI shielding and also for safety ground in line AC powered devices.

Earth ground 

Used in radio or power distribution systems, a connection to the earth itself. Also the other end of the connection for the safety ground, since the power line voltage will seek a path through the earth back to the power line supply station. This was the original usage of the word "ground", and the more modern meaning of the word would have been called a "floating ground".

The earth ground symbol and signal ground symbol are often interchanged without regard to their original meanings. As far as signal-level electronics (and this book) is concerned, ground almost always means a signal ground or floating ground, not connected to the earth itself.