NPCs are non-player characters. When given a gun via spawn menu, if they see an enemy (such as a Barney NPC seeing a Combine Soldier), they will attack the enemy. If the player gets too close they move out of the way and will refer to the player as "Gordon" (only if they are Half-Life 2 NPCs.) however this was removed, silencing NPCs. They are easily downloaded from the Toybox. They can only be given Half-Life 2 weapons by default - Crowbars, SMGs and AR2s being examples. They can have other weapons the player does not spawn with, such as the Annabelle and Stunsticks. They can be picked up but the player may see errors in place of weapons. Popular NPCs from different games are common creations. If you have Half-Life: Source, it will add NPCs from Half-Life 1.
Note
-Most NPCs can function of info-noded maps. This info node means that this node tells them were to go, and areas to avoid, they function like Navigation Meshes. But are easier to put in a map.