VBA is one of several emulators to emulate the Nintendo Gameboy Advance handheld console. This is a great achievement as the GBA has only been out for a couple of months.
I chose VBA as it seemed to be more complete that any of the others although as with any new emulator it's a little slow. As always as time progresses it'll speed up and become more compatible.
The Gameboy Advance if you don't know is a 32Bit version of the Gameboy. It probably has more in common with the Snes or Super Famicom as it can utilize 3D graphics and is full colour.
If you've had a Snes then you will see alot of the games crossing over to the new system. You can already get Super Mario and FZero and they play better than the Snes equivalents.
I'm getting seriously into GBA at the moment and I can't wait until Pokemon comes out.
Have Fun ;)
Update 10/01 - VBA has evolved yet again as it now supports the original Gameboy and Gameboy Colour. Also you will find that there have been significant speed increases and GBA compatibiliy is now very good.