An expendable NPC is a computer controlled character that has little if any value in terms of a game's story or plot. They are usually used simply to activate a trigger in a game or introduce a new part of the plot. Developers will sometimes give them generic statements to say during the game but they are rarely specific to that character.
Cpl. Johnston in Dead Space. One of the security officers who accompanies the player into the USG Ishimura. Shortly after making a casual remark about the damage to the ship, a necromorph comes from a vent above and gruesomely kills him. This shows the player what he or she is up against but has little real impact on the plot.
Reinforcements in first person shooters. Throughout series such as Call of Duty and Medal of Honor, the player receives aid in completing a mission from NPC soldiers who seemingly spawn endlessly. They often have poor AI and therefore are quickly mowed down. They are usually given names but don't say anything plot wise except for statements such as "Grenade!" or "Take that, Jerry!"