Sonar is an acronym for SOund, NAvigation and Ranging. It is also useful for navigation, communication or detection of other vessels.
Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes (Source). Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water (Source).
"A Sonar is a tool to map the ocan floor, find wrecks, find fish, find submarines, etc. It is our way of “seeing” things that we look for underwater at a distance. Sonar stands for: SOund NAvigation and Ranging. It is basically a “thing” that creates a sound at chosen frequency; The higher the frequency the better image; however, you loose distance. If you lower the frequency, then you increase the distance for the sound waves. The Sonar sends the signal (sound) away to the ocean floor and analyses the return signal (the echo) that has bounced off the seafloor or other objects on its path. Bats uses sound waves in the same fashion to navigate in the dark. They create a "visual picture" in their head by using their sounds. We humans do the same thing in the ocean when we map the ocean floor, but we use sonars to create a "visual picture" for us(Source)."
"In the picture [to the right] you can see how the ship uses sonar waves from two different sonars. One is a “hull mounted” sonar and the second is a so called “Side Scan Sonar” or as some people say “a tow fish”. The hull mounted sonar is fixed and sends the signals (sound waves) at a given frequency at a special angle. The “fish” or the Side Scan Sonar can be lowered to a specified depth and sends signals out on the sides of the "fish” in a an angle. The Side Scan Sonar can cover greater areas than a hull mounted sonar, which is a better choice at close range. The Side Scan Sonars come in different shapes, prices, frequency, etc. The lower frequency sonars are better for ocean searches where you have to cover vast areas. Higher frequency sonars (500 KHz and above) are better for close range searches or for smaller search targets (body searches, small boats, etc). In a side scan sonar the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the towfish to either side, typically to a distance of 100 meters. The strength of the return echo is continuously recorded creating a "picture" of the ocean bottom where objects that protrude from the bottom create a dark image (strong return) and shadows from these objects are light areas (little or no return). Think of it as the light shadow from lamp. However, it takes hours of practice to “read” the images from the side scan sonars. It is easy to miss details or to have wrong settings for the actual seabed. It takes many hours of fun practice to learn the skill of how to calibrate and read side scan sonar images(Source)."
Picture referred to in first paragraph.
Picture of the 19th century wood wreck "Storkyrka" in Sweden.