Myriad Grains operates on per-particle data passed through Maya DG graph. Computations are performed within following nodes:

mgAutomata – implementation of Myriad Grains Language interpreter. Core node of tool-set, performs transformations of points data according to the script defined in external file.
mgSampler – picks samples of given shading network and transfers the values to point fields. Utility node to cooperate with procedural textures and bitmap files.
mgMeshToPoints – decomposes mesh data into set of arrays.