Some really crap notes i wrote years ago, some stuff is slightly in accurate, but i've learned a lot since then... http://www.robthebloke.org/mayaapi/MayaApi1\_1.htm
Dependency Node == a node that has attributes that can depend on another. All nodes are dependency nodes (including dag nodes). A good example would be a material.
DAG Node == a transform or a shape. These are things that physically exist within the scene. They can be parented, but not in a cyclic way, i.e.
joint1 -> joint2 -> joint1
would be illegal.
A Dag path is the path to an object, for instance:
would be the path to joint3. A dag node may have one or more dag paths to it, for example:
There would be a single mesh node (i.e. one dependency node for it), but there are three paths to the mesh, given by the long names: (these are perfectly valid to use in mel!)
You can construct a dag path from MObjects if you want, i.e. (in psuedo code, but hopefully you'll see what i mean)
MFnDagNode is just a fuction set to ease access to a dag node. So, lets say i was to do something like this:
MObject o = getMobjectFromSomewhereFor("mesh1");
now, the function set has been initialised by an MObject to mesh1. But, MObject is basically just a direct pointer to the mesh. If i tried to get the world space matrix from that mesh, how does the API know which instance of mesh1 to return? The answer is that it doesn't, so it just fails and returns the local space matrix.
So, instead we can initialise the function set with a dag path, i.e.
now the function set knows which instance we are talking about, and can give us back some world space data. Uhm, if that makes sense up to now, then cool, that's more or less all you need to know about....
but, one last thing that can happen, but is extremely rare (the GUI doesn't let you do it, you have to do this with mel or the API)....
It makes a lot of sense with the shape nodes, but the same can also apply to transforms, i.e:
the above paths to joint1 are all valid, which is why if you did this:
MFnDagNode fn( joint1 );
you get the funcs:
fn.parent( instance_num );
uhm. hope that makes sense. In reality instanced transforms are extremely rare - infact i've never seen a Maya file with them unless i purposefully created one.