Different types of grounding
They are all connections to the same place, a point of zero voltage but with different purposes:

Signal Ground

This is a reference point from which that signal is measured, due to the inevitable voltage drops when current flows within a circuit, some 'ground' points will be slightly different to others. There may be several signal grounds in a circuit. Imagine if you had an amplifier with a voltage gain of 100 and you were amplifying a tiny signal, if the ground for the signal was elevated by just 0.01V the output would be wrong by 1V. Typically a signal ground would be a connection to the same stage of the circuit as the signal was connected.

Chassis ground 

This is the box or frame in which a circuit is built. Typically, it is ground to make a barrier between the user and the circuits inside to prevent electric shock or to shield against interference pick up or radiation. In some high current applications, it is used as a conductor to carry current, for example in a vehicle where running many thick wires to the battery would be impractical but making connection to the chassis is easy almost anywhere. Chassis ground should be connected to the other grounds, usually it is done at a point close to where the power source arrives.

Earth ground 

This is a theoretical zero. It is the potential of perfectly conducting Earth beneath your feet but obviously it varies widely depending on where you are. The connection to Earth is normally along your power providers cables or by a rod driven into the ground or sometimes both. It is supposed to be a return path for current in the event of a short circuit on your AC power lines but it is also used in radio applications as a 'zero' reference to read antenna signals against.