A cellar is a basic, unfinished basement. In a very old house, the cellar might have stone walls and a rough dirt floor.
Your cellar might basically be a basement, a place you keep your tools and do your laundry. Some cellars have specific purposes, including a storm cellar, where you take cover during a tornado or other storm, and a root cellar, where you store potatoes and other root vegetables during the cold months of the year. This kind of cellar comes the closest to the meaning of the Latin root, cellarium: "pantry or storeroom."