This site has a lot of info on food ingredients, what all that stuff means on the labels, and plenty of other info.

http://www.acreaturecomfort.com/ratingpetfood.htm

From the Glossary:
Meat By-products: Pet grade meat by-products consist of organs and parts not desired or not fit for human consumption. This can include organs, bones, blood and fatty tissue. It can also include brains, feet, heads, intestines and any other internal parts. Unbelievably, by-products can also contain cancerous or diseased tissue containing parasites, euthanized animals, .