Ultimately...when it comes to digital music these days, it is highly preferable you become familliar with these terms.
A VST is a plugin system © Steinberg Technologies. Many programs such as FL Studio, Sony ACID, Reason, Cubase etc. incorporate its ability to use such files. A VST is compatible (i think) with both PC and Mac, and depending on what plugin you find, it may be able to either 'alter' how a sound is supposed to output, or sample MIDI playback with its software synthesizer.
In practice, you may find a free alternative, or one which you have to purchase from.. In your case, I suggest start from the basics.
FL's already-included synthesizer generators could give you an idea on how a VSTi would work..though extras mean a possible wider variety of choices. (And if you're game, possibilities of creating your own samples..)
As for samples..i think you ought to stick with looking for extra soundfonts or something..but IMO, the majority are of 'lacklustre' and sound too much of like what a standard MIDI wavetable synth would appear to be..
You could always try looking for new samples on the web.. here's a site:
http://www.kvraudio.com/
EDIT (UPD): Latest WIP below this page.