An NPC, or non-player character, is a character just like the character you are playing. An NPC is often able to respond to your actions and the things you say. Bartenders serve drinks, thieves will pick your pockets, gangers will pick a fight over your kicks, and taxi drivers will even bug you for a tip!
The NPC is a character in the same story as you, treat them as you would another player-controlled character. They will notice snide comments, picked pockets and overheard whispers the same as another player will. When you encounter them, please don't treat them as if they don't exist.
Game masters "puppet" NPCs to bring the game to life and help player characters achieve certain things (or suffer certain consequences) in the game world. To learn what to expect and how to best get results from interacting with NPCs, there are a number of topics on the Sindome.org website's Forums with helpful information from admins and players: http://www.sindome.org/search/?q=talking+to+npc
@Notes can also improve your NPC and GM outcomes. Again, the boards: http://www.sindome.org/search/?q=%40notes
In addition to @notes, @request-puppet can be used to notify GM's and enter a queue for puppet requests. "help @request-puppet"