FM stands for Frequency Modulation, which is a method of encoding information onto a carrier wave by varying its frequency in proportion to the message signal. In FM, the frequency of the carrier wave is changed (modulated) based on the amplitude of the input signal. The frequency deviation of the carrier wave is proportional to the amplitude of the input signal, resulting in a wider bandwidth signal.

FM can be classified based on the type of modulation used, as follows: 

Narrowband FM: In narrowband FM, the maximum frequency deviation of the carrier wave is much less than the carrier frequency, typically less than 5% of the carrier frequency. This results in a narrow bandwidth signal that is useful in applications such as two-way radio communication and frequency synthesis. 

Wideband FM: In wideband FM, the maximum frequency deviation of the carrier wave is much greater than the carrier frequency, typically greater than 25% of the carrier frequency. This results in a wider bandwidth signal that is useful in applications such as FM broadcasting, where a high-fidelity audio signal is required.