There is a MIDI standard for sample libraries as well, called DLS (downloadable sounds). Soundfonts can be used with Linux as well, but not with Macs because the hardware is not compatible.
Soundfonts are samples stored in files with the extension SF2. You will need a soundfont compatible soundcard, most likely a Sound Blaster of some sort. Virtual instruments run on Windows or Mac.
They are synthesized, but the output is great. These plugins are used in Cubase and Acid, and are literally virtual instruments.
If you're looking for virtual instruments, then I strongly recommend the Native Instruments VSTi plugins.
