A frittata is an open-face omelette originating from Italy. Unlike a traditional omelette where the eggs are cooked at high heat and then folded, a frittata is typically made by sauteing some combination of vegetables and meat until softened, and then the egg mixture is poured into the pan and then left to set at low to medium heat. A non-stick brunch pan such as the one in the photo is most ideal, but any pan which has been sufficiently seasoned with butter or oil will do.
After the vegetables and/or meat has been cooked through, it should take about seven minutes give or take for the eggs to set properly. About half way through the setting process you can add cheeses such as feta to the mix.
I usually just end up using whatever is in my fridge, but normally I will put in spinach, peppers, tomatoes, onions, and then maybe ham, bacon, and feta or cheddar at the end. It takes about four eggs worth of liquid to fill this brunch pan, but use whatever you want that reflects the number of people you are cooking for. You can serve this by cutting into slices.