First let me tell you im using Logic , and i can tell for a long time i never believed in VST instruments, ive been doing music for a long time, well about 3 yrs ago i started using them, i can tell you they are awsome, i hardly use my Korg triton, ive been using all sorts of plugins, the sounds are better that alot of outboard gear. Iv been i true believer.
Now for your ? Vst are the same thing a having keyboards and drum machines in you computer,once installed they are at your command, say you want some good guitar sounds, you can buy say.... plugsound http://www.ultimatesoundbank.com/usb/exec/plugsound2
check this web site out and you can hear for yourself when you get to the site just click on the demo and you can get a sample of what the can sound like, just plugsound alone has 6 volumes, by using say the plugsound sound that i sent you is like having hundreds of guitar sounds from just one volume, and this is just one of hundreds of vst sounds, tat you can buy, you'll have to do research though to see what you want.
I know im kind of rambling but its kind of hard hard for me to explain.Email at Rence12@aol.com if you have any more ? about vst, id be glad to help...