A non-combat unit is a unit that cannot attack like a military unit, but benefits your civilization in some other way. There are a total of five kinds of non-combat units: the Settler (which can found new cities or increase the population of existing ones), the Caravan (which can establish trade routes with other civilizations), the Spy (which can steal from or interfere with operations in enemy cities), the Militia/Ranger Militia (these however, can still be used for scouting or defending), and Great People that can provide bonuses to your cities.