Nice post on M, Quadrant and Repository via xosfaere. Here is a snippet.
Then we have MGraph. Now this is the simplest part because this is simply an abstract datamodel, just as the XML Information Set (Infoset) is for XML. It is a so-called labelled Directed Acyclic Graph (DAG) which means it is more structurally powerful than the tree-based Infoset model. MGraph is the result of a parse of an M instance file combined with an MGrammar file (you need the language syntax and MGraph mapping in order to create an MGraph out of an instance file). So this could be, for example a list of contacts in a text file and then the MGrammar file will specify the contact syntax and how it maps to an MGraph.
