Amateurs radio operators have a variety of modes to choose from when engaged in two way communication. A mode refers to the way the signal is modulated during transmission. Commonly used forms of modulation are AM, FM, SSB, and digital. In order for a signal to be transmitted and received in a readable manner it is modulated electronically. Both transmitter and receiver must be using the same form of modulation for the communication to be successful. Each of these modes will be discussed below. The table of preferred modes for voice communication gives some idea of what to expect when you use a particular band. Some modes such as RTTY use LSB for all bands.
Each mode has its own unique characteristics. One of these is amount of bandwidth occupied by the signal. CW is quite narrow (less than 250 Hz) while FM is rather wide (15-20 kHz). A narrower signal means there is room for more signals and thus more activity on the band. On the other hand a narrow signal transmits less quality or information. CW requires the use of Morse code whereas FM results in a high quality signal for voice communication. In the following each of the more widely used modes is discussed briefly.