VST (Virtual Studio Technology) allows the integration of virtual effect processors and instruments into your digital audio environment. These can be software recreations of hardware effect units and instruments or new creative effect components in your VST system. All are integrated seamlessly into the host application. VST was developed by Steinberg Media Technologies in 1996 and since then it become an industry standard. Nowadays many sequencers and audio editor utilize VST as a primary interface for supporting third-party processing plugins.
VST is basically a software interface for exchanging Audio data between the host and plugin. Host application handles all tasks of Audio input, output and routing, but the plugin only process the pure Audio samples applying one or another effect. Modern VST specifications also include specifications for MIDI processing and MIDI synthesis (VST Instruments) as well as some other specific uses.
WIDI VST Plugin is a plugin that receives Audio input from the VST host and outputs MIDI data either by VST interface back to host or directly to the MIDI Out Device or even to a MIDI File.