A type of bacteria. E. Coli includes a large group of bacteria. Some are harmless and others can make us sick.
Bacteria. Escherichia coli is a bacteria that usually lives in the digestive tract of humans. Most of the strains are harmless. There are some strains that are harmful that get into our food especially meat if not cooked well, water contamination from sewage or fecal material, or infected people not handwashing well and contaminating our food supply.
Common gut bacteria. E. Coli is a bacteria that is normally found in the intestines. There are "good" and "bad" strains of this bacteria where the good ones live harmlessly in the intestine; the bad ones can cause symptoms of pain, bloody diarrhea, etc. In addition, when either the good or bad strains are found in other parts of the body (bladder, blood stream, etc.) they can cause disease.