Ramekins are small baking dishes about 3 to 4 inches in diameter holding 4 to 6 ounces with the 4 ounce dish being the most common dessert serving size. Ramekins are used for baking custards like creme brulee or individual souffles, gratins. The cups are typically porcelain or earthenware and should be ovenproof. The typical 4 oz ramekin is about 3 1/2" wide by 1 1/2' deep.
Ramekins can be found in most kitchen or big box stores including Bed Bath and Beyond, Target or Amazon.com.
If you don't have ramekins, you have several good options: