VST stands for (V)irtual (S)tudio (T)echnology. VSTs offer a way to insert small, third party, programs, called plugins, into music software like BiaB and RB. For detailed information about VSTs see this wiki page
CamelCrusher from Camel audioEdit
Possibly the best VST around for guitars. This plugin will make your midi guitars sound like the real thing and your audio guitars scream. There are 20 presets to get you started. If CamelCrusher can't do it then it probably can't be done. Five stars.
California Sun from AuraPlugEdit
This is a great little amp sim for you guitars. It has a clean and dirty mode and controls for level, drive, Low and high. Simple and sounds good, especially the dirty mode.
A truely great amp simulator. Dirthead has three presets, clean, crunch and hi-gain. Using those as a starting point you can adjust tone, voiceing, gain and volume. There is also a cab switch.
Dirthead is simple, uses very little processor power and sounds good. Highly reccomended.