Units that cost only food and wood. The main lines are Spearman, Pikeman and Halberdier; Skirmisher and Elite Skirmisher; and Scout Cavalry, Light Cavalry and Hussar. Some civilizations add their own trash units or variants.
Late in long games the gold on the map runs out and trash units become most of each army. They are weak against most things one on one, but each one counters a group of gold units: Pikemen against cavalry, Skirmishers against archers.