There are two types of characters: Player Characters (PCs, or simply "players") and Non-Player Characters (NPCs). A Player Character is an avatar in the game environment that is controlled by a player, whilst Non-Player Characters are controlled by the game software and can only interact with PCs through scripted events or artificial intelligence (AI). There are three types of NPCs. Friendly NPCs cannot attack friendly characters and vice versa. Hostile NPCs are either of the opposing faction or are mobs (enemies controlled by AI) and will freely attack any PC with whom they are hostile. Neutral NPCs will only attack if provoked. Some NPC interaction is affected by the reputation a PC has with them. NPCs in major and minor cities can buy and sell merchandise, train class and profession skills, give quests and provide a large number of services that are needed in the game. While some will merely offer advice or further the story, others, such as city guards, patrol around set paths to keep cities defended against attacking PCs or hostile NPCs that may attempt to invade. Guards can also provide directions to common locations within their city. When creating a character in World of Warcraft, the player can choose from ten different races in two factions: Alliance and Horde. Race determines the character's appearance, starting location, and initial skill set, called "racial traits". The Alliance currently consists of Humans, Night Elves, Dwarfs, Gnomes and Draenei. The Horde currently consists of Orcs, Tauren, Undead, Trolls and Blood Elves. Draenei[10] and Blood Elf characters were introduced in The Burning Crusade, and require that expansion in order to be created. In addition to the ten playable races, there are many NPC races including (but not limited to) Goblins, Ogres, Murlocs, and Naga.