The Maya API uses a convention of prefixes on its classes to distinguish the various types of C++ objects that it uses.
Any class with this prefix is a function set used to operate on MObjects of a particular type.
These classes are iterators and work on MObjects much the way a function set does. For example, MItCurveCV is used to operate on an individual NURBS curve CV (there is no MFnNurbsCurveCV), or iteratively, on all the CVs of a curve.
Classes with this prefix are all , API classes designed for you to derive from and create your own object types.
Most of these classes are , though there are others. For example, is a handle on Maya’s internal objects, and MGlobal is a class of static methods that operate globally and do not require an MObject on which to operate. (See for information on MGlobal.)