What is MAS ? How is it different from VST?

MOTU Audio System (MAS) is the software 'engine' that turns your Macintosh into a multitrack audio recording environment, complete with mixer, bussing, and real-time effects processing. MAS is a system extension which uses your existing CPU to accomplish what used to take multiple DSP chips and special acceleration hardware.

While MAS has been designed by MOTU, VST technology has been developed by Steinberg.

Virtually, both (MAS and VST) perform the same function : receive MIDI input and transform it into audio output. Basically, these technologies are bound by their host (sequencer), not by the platform. VST and MAS have hosts on both Windows and Mac. You can still use VSTi in MAS compatible softwares using a VST Wrapper from Audioease (www.audioease.com). In virtually all respects, using a MAS synth is the same as using VST Wrapper to run one in VST format.