I am experimenting with material in the blender render engine and i can't seem to figure out the real difference between the two '+' signs when adding materials.
One adds a new material (copying the current material):
This will replace the material in the active slot. The first material will still exist (and can be added to an object or objects), at least until the .blend is closed. Then unused datablocks are cleaned up.
The other adds a new material slot:
Material slots are places where other materials can be added to the same object data.