A projector is used to emit a pulse of sound energy. The accoustic waves travel outward and rebound to varying degrees from objects in the field. The reflected wave is then picked up by a hydrophone. Modern sonar systems use piezo-electric or magnetostrictive components both to produce the ping and measure the very small pressure variations associated with the return. This is refered to as active sonar.
Passive sonar is simply the receiving half of the active system.
Figure 6 shows a modern acoustic array. Even from a very simple one-component echo sounder system such as this, a number of data can be found. The frequency shift of the returned signal can be used to determine the radial velocity of the target. Thirdly, the strength of the returned signal can help guess the properties of the target. Larger cross-sections will yeild larger returns, however, different materials will reflect differently.