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One of the problems once faced by wanna-be MIDI musicians was that humans read musical notes differently than MIDI sound modules. To oversimplify, we use letters to represent each note (“C sharp in the fourth octave, for example), and MIDI sound modules use numbers. Since most MIDI sound modules can play up to 128 notes, a MIDI sound module will number them from 0 to 127. The problems arose when different manufacturers started using different numbers to correspond with different notes. A C sharp in the sixth octave might correspond to number 61 on sound modules made by one manufacturer, and to a 49 on one made by another. We're talking serious chaos here – imagine what it would do to your composition if you made it on a Roland and tried to play it on another manufacturer's sound module. Even worse, some very disorganized manufacturers assigned different numbers to different notes even in the same sound module depending on which instrument you played. In other words, you just about needed a degree in computer science to figure it all out, and composing a song meant about 3 times as much grunt work as actual composing.
The General MIDI standard organized this musical chaos by decreeing that all GM –compliant patches must play an A440 pitch in response to a MIDI command that included the MIDI note number 69. All other MIDI note numbers were calibrated according to this standard so that the same note number would play the same note on any GM-compliant sound module, regardless of who manufactured it. Drum sounds were similarly standardized; with 48 MIDI note numbers standardized to correspond with 48 particular drum sounds. For those of you interested in a bit more detail, MIDI Channel 10 is reserved as the default channel for drum sounds. So as long as you pound out your drum parts using the GM standardized codes, and be sure to use MIDI channel 10 for the drum parts of your composition, you shouldn't be in for any nasty suprises when you try to play your composition.
The problems that electronic musicians faced with playing their compositions on equipment made by different manufacturers was a serious one in the 1980s. Hook up a MIDI Controller made by one manufacturer to a sound module made by another manufacturer, and your flute solo could come out as a drum solo. You could try adjusting the volume and end up changing the pitch instead. This is because MIDI commands, which are used to control every aspect of the composition from notes played, instrument used, volume, pitch, and many other parameters, are numerical, and once upon a time (meaning the 1980s) different manufacturers used different functions to correspond with different MIDI Command numbers. For example, the number corresponding to a trumpet sound on one brand of equipment might correspond to a harmonica sound on another brand of equipment.
There were many other problems as well, most of them arising from a lack of standardization of the correspondence between MIDI Command numbers and the actual parameters that they adjusted. For this reason, the General MID (GM) standard was created – so that all (or most of) the numbers used to generate any particular MIDI command would do the same thing on any brand of equipment that incorporated the General MIDI standard – for example, the number 12 placed at a certain point in the string of digits that represents any MIDI command now triggers any GM standard sound module to play a Vibraphone sound, and nothing else. This sound may differ somewhat on different sound modules (sound quality will vary depending on how expensive the sound module is and what kind of technology it uses), but at least you won't end up playing a flute instead of a vibraphone.
The GM standard incorporated a variety of standardizations other than MIDI commands – for example, it required all GM compliant sound modules to be fully multi-timbral – that is, each sound module had to be able to receive MIDI messages on 16 different channels, so that the sound module can play 16 different “patches” (equivalent to 16 different instruments) at once, corresponding to the 16 available MIDI channels.