VST2 was an update of VST, while VST3 was more of a ground up overhaul of the entire platform. The differences are mostly developer side.
As VSTi virtual instrument technology was under development at Steinberg, a platform for virtual instruments using DirectX engine technology was being. It includes a preview version of VST3 for Linux platform, the VST3 part of the SDK gets a dual license: 'Proprietary Steinberg VST3' or the 'Open-source GPLv3'.
