Audio Stream Input/Output (ASIO) is a protocol allowing communication between a software application and a computer’s sound card. This was developed by Steinberg back in the 1997. It provides a low-latency and solves synchronization issues between the different input and output channels.
VST 3 SDK
With this VST Developer Forum we hope that communication between VST Developers will be improve and that it will answer your questions concerning the development of VST 3 Plug-ins and/or VST 3 host. You can find now online the VST 3 portal: https://developer.steinberg.help/display/VST
VSTGUI is a user interface toolkit mainly for audio plug-ins (VST, AAX, AudioUnit, etc).