It is SOund NAvigation and Ranging. Sound is used to navigate, and to detect objects in water. Sonography also has other applications.
There are two main types of Sonar systems, active and passive. In active sonar, a pulse of sound is sent out and the operator waits for echoes. How it works is that, when a sound signal is sent into the water, part of it will be reflected back when it hits an object. The distance to the object can then be determined by measuring the time between when the signal was sent and when the echo is received.
In passive sonar, the operator listens to sounds emitted by the object one is trying to locate. Passive sonar uses the sounds emitted by objects such as ships, submarines and creatures such as marine mammals and fish to determine their location.
We also find sonography used in medicine and industry. The sonogram is a broadly applied investigative medical tool that is non-invasive and wonderfully effective. We also see sonography used in industry to look "through" solids for defects or flaws.