If players are the stars of the show, then NPC's would most certainly be the supporting cast of extra's, whose job it is to fill every street, starship and tavern from Rigel VII to Nimbus III. Without NPC's the world in which the players characters existed would be very dull indeed. NPC's come in two sorts, those which the Game Master has created and introduced in to the story for the purpose of helping or hindering the players, or those which the players themselves have created. This could be anything from a friend the players character speaks to or a competative rival, whom the player uses to provake their character. NPC's are a useful tool, they can extract information from the players character, via converstation or they can bring information to the players attention which they otherwise might not have access to.

As a rule, players can be just as protective about their NPC's as they are their main characters. So it is always adviseable to consult a player before writing their NPC in  to one of your posts.