Haggis is normally made with sheep's 'pluck' (heart, liver and lungs), minced with onion, oatmeal, suet, and seasonings, and traditionally boiled in a sheep's stomach.A lot of haggis is made these days using artificial casings. There is a good article at Wikipedia if you are interested:

http://en.wikipedia.org/wiki/Haggis

Note that it is often made with pork rather than mutton. This is good for me as I'm allergic to lamb. When I was a kid my parents used to get the haggis made for the Burns Night dinner and it was delivered in huge tinfoil roasting trays. They always ordered an extra one and split it up, freezing large portions that we could defrost over the following months for special meals. Yummmmm.