There are three ways to play MIDI music with PC. They are using FM Sound Generator, using Software synthesizer, and using External Sound Generator. Sound Generator is a device that receives MIDI command and generates corresponding sound. MIDI command describes sound to generate, such as "Play note C with piano tone and make it length of 4th note", while actual sound is waves of air. Therefore MIDI command has to be transformed to wave of sound. Sound Generators do this process for you.  You can think of WAV file of Windows. WAV file is recorded as wave of sound while MIDI is recorded as commands. These differences result in file size. Command that says "Play note C with piano tone and make it length of 4th note" needs several bytes while recording piano sound for a few seconds many bytes.

Each Sound Generator has a different method to generate sound from MIDI command. FM Sound Generator is embedded in PC's Sound Card. FM Sound Generator takes a MIDI command, then modulates simple wave (such as sign wave) and make the sound. Imagine the following situation. You draw a line on paper randomly and you can regenerate the line using some polynomials and parameters. It may be not perfect, but it could be good job if you spend some time. The sampling method that will be shown later generates better sound, but good point of FM method is that you can easily modify original sound. In the case of former example, you can change parameters and get different sound.  Software Synthesizer and External Sound Generator have (PCM) sampling sound of music instruments. It receives MIDI command and regenerates sound from sampling sound. Software synthesizer uses PC's CPU when it converts MIDI command to actual sound wave while External Sound Generator uses its own CPU when generate sound. (External Sound Generator is connected to PC with MIDI cable.) Even if it is a board that is in PC, if the board has its own processor and memory, it is equivalent to an External Sound Generator. When software synthesizer is used, if CPU power is not enough, the generated sound often breaks. External Sound Generator has many prepared tone and many functions, so it is the best sound generator.  The method of Analogue synthesizer is that it combines waves such as sign wave and square wave, and generates required sound. In the case of former example, to reproduce random line, some polynomials can be combined. It is hard work to reproduce the original line.

Sound Generator Price Quality  FM Sound Generator free (embedded) Low  Software Synthesizer cheap (less then $100) Middle External Sound Generator expensive (around $500) High

Links for software synthesizer

Links for external sound generator