SoundFonts are similar in idea to regular fonts (TrueType in Windows). An ordinary font will take a piece of text and give it a different look and style. A SoundFont will take a piece of MIDI music and give it a different sound by replacing some or all of the MIDI instruments with new sounds. A SoundFont can be something as simple as a new wavetable instrument set, or as complex as the vocals for a song. As far as I know, only professional sound cards can use SoundFonts. There are a few programs which use ordinary SoundFonts as the wavetable bank for a wavetable emulator, too.
Of course, that does not mean that other cards can not use things somewhat similar to SoundFonts. Many newer sound cards allow you to load new wavetable sound patches and have their own SoundFont-like files. MidiRenderer can convert MIDI to MP3 using SoundFonts.
There is a great number of SoundFonts on the internet. Some of them are free, some are not.
Papelmedia have some of the best orchestral and vocal soundfonts: http://www.papelmedia.eu/english/download2.htm There is the best "Aah" choir you can get for free.
SoniMusicae have some of the best piano and old Baroque/ Renaissance instruments sampled and most of them have a soundfont version: http://sonimusicae.free.fr/accueil-en.html
SonicImplants have some free trials, that means soundbanks stripped of some sounds. Still, top quality and usable: http://www.sonivoxmi.com/ProductList.asp?new=1&lstSort=Synth&mnu=List&lstShipCode=D&optDetail=D&lstSynthSearch=Soundfont
Many of my sample set are available in the sf2 or sfz formats at http://lesproductionszvon.com/products.htm