welcome!

//note for maya c++ API

MObject
Access to all Maya objects (curves, surfaces, DAG nodes, dependency graph nodes, lights, shaders, textures, etc.) is done through a handle object called an MObject. This handle provides a few simple methods to help determine the object type (see the MObject class documentation for a complete description). The MObject destructor does not delete the Maya object that it references—calling the destructor only deletes the handle object, thereby maintaining ownership.

Wrappers
Wrappers exist for simple objects such as math classes (like vectors or matrices). They are generally fully implemented C++ classes with public constructors and destructors. API methods may return a wrapper, which you are then responsible for—leaving scope will usually be adequate for deleting the wrapper. You are also free to allocate and deallocate them as necessary. In the previous example (Building a curve using a plug-in), MPointArray and MDoubleArray are wrappers. You always own the wrapper that you reference.

Function sets
Function sets are C++ classes which operate on objects. In the example, Building a curve using a plug-in, MFnNurbsCurve is a function set (the MFn prefix indicates this).

Proxies
The Maya API uses proxy objects to create new types of Maya objects. Proxies are objects that you create but Maya owns.

Naming Conventions
The Maya API uses a convention of prefixes on its classes to distinguish the various types of C++ objects that it uses.

MFn
Any class with this prefix is a function set used to operate on MObjects of a particular type.

MIt
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.

MPx
Classes with this prefix are all Proxies, API classes designed for you to derive from and create your own object types.

M classes
Most of these classes are Wrappers, though there are others. For example, Function sets 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 Chapter 2, "Selecting with the API" for information on MGlobal.)

MFnNurbsCurve curveFn;
MObject curve = curveFn.create( ... );
curve = curveFn.create( ... );

The global active selection list can be copied through:

MGlobal::getActiveSelectionList()

This returns an MSelectionList and makes a copy of the list.

Any changes you might make through MSelectionList methods will not affect the global list unless you use:

MGlobal::setActiveSelectionList()

You can also create your own lists using MSelectionList to merge with other lists, including the global list. You can use this list to create sets of objects (see setObject() method).

MSelectionList
MSelectionList provides you with methods to add and remove objects from the selection list, as well as walk through the objects on the list.

Simple plug-in example
#include <maya/MSimple.h>
#include <maya/MGlobal.h>
#include <maya/MString.h>
#include <maya/MDagPath.h>
#include <maya/MFnDagNode.h>
#include <maya/MSelectionList.h>

MStatus pickExample::doIt( const MArgList& )
{
    MDagPath            node;
    MObject             component;
    MSelectionList      list;
    MFnDagNode          nodeFn;

    MGlobal::getActiveSelectionList( list );
    for ( unsigned int index = 0; index < list.length(); index++ )
    {
        list.getDagPath( index, node, component );
        nodeFn.setObject( node );
        printf("%s is selected\n", nodeFn.name().asChar() );
    }

    return MS::kSuccess;
}
DeclareSimpleCommand( pickExample, "Alias|Wavefront", "1.0" );

MItSelectionList
MltSelectionList is a wrapper class containing selected objects. This can either be a copy of the global active selection list, or a list you build yourself.

MItSelectionList lets you filter the objects on the selection list to only see objects of a particular type (MSelectionList does not let you filter selected objects).

MGlobal::getActiveSelectionList( list );
for ( MItSelectionList listIter( list ); !listIter.isDone();
listIter.next() )
{
    listIter.getDagPath( node, component );
    nodeFn.setObject( node );
    printf("%s is selected\n", nodeFn.name().asChar() );
}