NPC's play a vital role within the roleplay. While the term has been made popular and been quite frequently used within videogames, NPCs do differ somewhat. This page provides some insight as direction to known NPC's.
Meaning of NPC
The traditional meaning of NPC is "non-player character", indicating that they aren't directly controlled by the player or writer. This often means that they don't enjoy the same amount of attention for development, plot (armour) or abilities that the "player-characters" tend to possess. Exceptions are usually dangerous villains, mentors and other figures with prominent roles within a story or scenario.
The difference with NPC's in the Roleplay (this one, that is) is that they aren't simple scripted or lesser characters by default. The term NPC is being used for characters that usually don't have a character sheet as large or detailed as a 'player-character' nor appearing that often in a post/story as the 'player-character'. It is highly unacceptable to just murder an NPC that has been made up or has been consistently controlled by another writer/player.
There are a lot of NPC's involved within the roleplay. It isn't mandatory that you know every single one of them. But it might prove interesting to read up on a few of them. There is a large variety of NPC's and some have their own CS's due to various reasons.