History and File Format
SoundFont is actually a file format specification that used to be attached to the Emu EMU8000 chip, which is used on many PC soundcards. Emu cleverly detached the file format specification in version 2.0, trying to encourage other manufacturers to make equipment that would use SoundFont as their native file format. That almost happened, but in a backwards sort of way. Many people, mostly amateurs since that is the basis of the soundcard market, started sharing and making available their SoundFonts, so there became a sizable library of them. Then, other developers of other sound-generating equipment and software saw it as marketable to include SoundFont reading capability on them - Reality and Reason are a good examples of this. SoundFont files carry the extension .sf2 although SoundFont 1.0 files have the extension .sbk. Translator does not deal with SoundFont 1.0 so that will be exempted from this discussion.

A SoundFont .sf2 file is self-contained, carrying all the parameters and wavedata it needs. No pointers are used to external files.

Apple, rising again as a valid computer company, adopted SoundFonts into QuickTime 5 and later. This is a great boost to the format, as Apple needed this type of file format-operating model.

A good example of the portability of SoundFont is that it is the only way to get into Reason NN-XT. Since Propellerheads does not release the file format of Reason (it is encrypted and thus is not reverse-engineerable by mortals), this is the only way in.

Architecture Description
A SoundFont is capable of several things - it is unique because whatever you need in the file, you can include and specify. If you do not, it does not need to be included in the file. However, this does not mean a SoundFont is capable of EVERYTHING. The SoundFont shows its heritage of being attached to the EMU8000 by offering a limited amount of parameters - the envelopes in particular are primitive. It is not a good idea to consider a SoundFont the universal file format that can conceivably carry every parameter and hardware platform offered.

The first level of the SoundFont is a Preset. A Preset can reference one or more Instruments. An Instrument can reference one or more Zones, which are keyranges with a certain set of properties. The SoundFont format is clever as it doesn't require all the parameters to be included in the file - if a parameter entry is missing, it simply assumes the default.

Another interesting aspect of the SoundFont is that parameters can be modified up the chain. For example, you can have three Instruments with certain settings, and then program a Preset that combines all the three Instruments plus can implement it's own envelopes, parameters, and pitch changes to override or add to the Instrument level parameters.

The basic Instrument unit can either be the Preset or the Instrument, depending on the programming. If an Instrument is the "Instrument Unit," there usually is a matching Preset that maps just the one Instrument.

Comments
A SoundFont, upon observation, is quite a capable format. If it weren't for company rivaleries, it would probably be what all samplers loaded.