The official flag of the United Kingdom. The Union Jack is primarily a land flag and should not be flown at sea, where a red ensign should be flown instead. These still look great on a wall or in a garden though!
Flying notes - the Union Flag is not quite symmetrical and is commonly flown upside down by mistake. The correct orientation is as shown, with the white saltire slightly higher than the red cross of St Patrick on the side of the hoist (the side with the toggle). If fixing to a flat surface like a wall, the hoist should be mounted on the left.