A Non Player Character or NPC is basically a character that anyone can use. The NPC can be frequently or infrequently used based on his/her position within the given faction.

While as stated above, NPC's can be used by any player, Staff members are the only ones who can use Canon NPC's.

Canon NPC's are characters who were featured on the original Knight Rider or the 08 series, but were not regular cast members such as characters Dylan Fass and Danny Clark.

NPC's also do not have to be canon, they can be completely original and anyone can create an NPC by using the NPC template located on the TKR:TNG forum.