This specific type of amplifier was developed to be used in applications that consume low voltages. Though the gain is fixed at 20 but due to the presence of an external capacitor and a resistor, the gain rises up from 20 to 200. Some of the notable features include Battery operation, minimum external parts involvement, the voltage supply range is wide i.e. 5V-18V or 4V-12V. The inputs are ground referenced, it is available in 8-pin MSOP package etc. Its main applications are in FM-AM radio amplifiers, intercoms, TV sound systems, Ultrasonic drivers, power converted, etc. You can download LM386 DATASHEET from the given link.