Haggis is a savory pudding containing sheep's pluck, which is the heart, liver and lungs of a mature sheep; minced with onion, oatmeal, suet, spices, and salt, mixed with stock, and traditionally encased in the animal's stomach and simmered for approximately three hours.
Most modern commercial haggis is prepared in a sausage casing rather than an actual stomach.
As the 2001 English edition of the Larousse Gastronomique puts it, "Although its description is not immediately appealing, haggis has an excellent nutty texture and delicious savory flavor".