Bifrost has evolved from Autodesk’s purchase of Exotic Matter, creators of the fluid simulator Naiad. Since the acquisition we’ve seen Bifrost go from fluid simulator to a much more rounded tool, rapidly chasing the heels of simulation and node based packages like Houdini and ICE.
The updates in 2.1 give users new nodes for geometric queries, improved node graph interface, larger aero seams, and many FX updates. For a complete list see the Autodesk blog post.
An interesting article with Exotic Matter co-founder Marcus Nordenstam and his journey from Naiad to Bifrost can be found on Befores and Afters.