mb=Maya Binary, native binary file format used by Maya. This is the default file format.
ma=Maya ASCII, the native ASCII file format used by Maya.
ASCII file
A text file in the American Standard Code for Information Interchange (ASCII) format. Maya scene files can be saved in an ASCII format or in a binary format. The file extension for Maya ASCII files is .ma; the file extension for Maya Binary files is .mb. Maya Binary files are usually smaller and load faster than Maya ASCII files. Maya ASCII files have the same information as Maya Binary files, and you can edit them using a text editor
==!!!===Maya ASCII files have the same information as Maya Binary files, and you can edit them using a text editor==!!!===