A unit in Civilization V is a mobile entity which can move around the map and perform certain functions. Most units in the game are military (combat) units - they have the ability to attack and damage other units and cities. Those that can't do that are also known as civilian units. Units are the "mobile" part of your empire, just as cities are its "stationary" part. Civilian units are essential to its economy, while military units ensure its safety and allow you to attack and defeat other empires.