Black Pudding is a sausage made from blood and filler, cooked until it congeals. The blood is usually cow or pig and the filler can be barley, oatmeal, fat, meat or some combination of these.
It can be eaten cold, or boiled or fried, and may be served battered in chip shops, especially in Scotland.
Black puddings come in various shapes and sizes - A traditional Bury Black Pudding is a horseshoe-shaped sausage about 10cm across and will usually be served whole, whereas ones for catering use come as a straight sausage about 6cm in diameter and a good 30cm long and are sliced and fried. This picture is of one of the large catering-size black puddings.
A standard component of the Full English (and Scottish and Irish) Breakfast is a fried slice or two of black pudding.