Like MATLAB, MRST is not primarily a simulator, but is developed as a research tool for rapid prototyping and demonstration of new simulation methods and modeling concepts. To this end, the toolbox offers a wide range of data structures and computational methods you can easily combine to make your own custom-made modelling and simulation tools. MRST nonetheless offers quite comprehensive black-oil and compositional reservoir simulators capable of simulating industry-standard models and also contains graphical user interfaces for post-processing simulation results. The software has a large international user base and also includes third-party modules developed by researchers from all over the world.
For more information, see the MRST website.