NPC's or Non-Player Characters can be extremely helpful. Most of the time an NPC is needed to further a story. In the classical RP sense of Dungeons and Dragons, an NPC often facilitates a party going on a quest, sells them the gear they need, or provides a nice boost out of the tavern if someone is drinking too much. They are temporary disposable characters, usually without the benefit of a sheet or a bio.
For NPC's in use, see the Category:Non-Player Characters and reference the Non-Player Character Guidelines for how they can fit into an existing story.
NPC vs. PC
The differences between an NPC and a PC can get muddled. Especially, as often happens, when an NPC is used frequently for the same task. The main difference is in depth of character and plans for a character. An NPC should be a character that is purpose oriented. That is to say, they were created to do a task. Be that, facilitate the introduction of a new character, be a nurse in a hospital, the bartender of a bar, etc.. Overall NPC's are minor characters in any plot and serve the purpose of making sure the player characters get where they need to go.
Blazing Umbra
In our setting, NPC's may be used for this purpose without character approval, but one should be wary about back-dooring a character into play and should not introduce an NPC that doesn't fit the setting.