I am surprised I didn’t encounter (or perhaps realize their importance) these magical things until today. Not that I have been making music forever, but I just cannot believe how much I had been missing.
Well, essentially, these are sampled sounds put together into a patch/bundle/bank. The sampled sounds within a soundfont can be controlled through MIDI CC making them something that resides halfway between synthesized and sampled sounds. E-MU came out with these first, and still distributes professional hi-fidelity ones. More details here.
These soundfonts usually come in a package that represents an instrument. A Cello soundfont e.g., may have all the Cello notes sampled as individual waveforms put together into a single soundfont.
You can then load these soundfonts into a soundfont capable VST like the freely available SFZ from RGC Audio.
Hammersound has a whole bunch of collected soundfonts which can be downloaded free of charge. I just downloaded the Cello soundfont which I plan on using in my track I am presently working on. I just absolutely love Cello for its sad and gloomy timbre :).
Soundfonts are ideal when you need a more natural sound than a synthetic one.
HTH