Originally Posted by
TuggyNE
Emmerask has a lot of it, but the root distinction that I started with is that a DM-PC is one where the DM says something like "hey guys, I was thinking of running a PC because {I want to play too|you guys are short-handed|it's custom to run a DM-PC|whatever other reason" and then the rest of the group says "OK, whatever" or "sure, sounds great!". A TN-PC is one where the DM brings in an NPC that turns out to be, in all ways but name and person running it, a PC; generally in this case there's little or no prior approval by the group as a whole. If you talk to the DM about it, a DM-PC will be described as a DM-PC, but a TN-PC will be described as a totally legitimate NPC.
I'll see if I can condense that down in the OP to make the point a bit clearer.