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.
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.)