On MIDI Sound modules (i.e., whose Patches are instrumental sounds), it became desirable to define a standard set of Patches in order to make sound modules of sound cards and synthesizers more compatible. For example, it was decided that Patch number 1 on all sound modules should be the sound of an Acoustic Grand Piano. In this way, no matter what MIDI sound module you use, when you change to Patch number 1, you always hear some sort of Acoustic Grand Piano sound. A Standard was set for 128 Patches which must appear in a specific order, and this standard is call General MIDI (GM). For example, Patch number 25 on a GM module must be a Nylon String Guitar.

A GM sound module should be multi-timbral, meaning that it can play MIDI events upon all 16 channels simultaneously, with a different GM Patch sounding for each channel.

If the GM module also has a built-in "drum module" (ie, usually one of 16 Parts), then each of that Drum Part's MIDI notes triggers a different drum sound.

Newer sound cards such as the Soundblaster AWE 32/64, and synthesizers, have more than one bank of Instrument Patches which can be selected, however most MIDI files downloaded from the Internet expect a sound module set to General Midi. 

The GM standard also defines a few global settings (Continuous Controllers) that the sound module should respond to. Check the MIDI implementation chart for you specific module, since not all sound cards respond to all of the Control changes, particularly Controllers 91 and 93, which are NOT included in the Level 1 specs.

1.     Modulation
7.     Volume
10.   Pan (stereo position)
11.   Expression
64.   Sustain
91.   Reverberation
93.   Chorus effect
121. Reset all controllers
123. All notes off

For further details refer to the General MIDI Standards and specifications.