Crohn's Disease affects the digestive tract, it is one of many Inflammatory Bowel Disorders. It is often called the "gum to bum" disease as it can occur anywhere from the mouth, down the esophagus into the stomach, in the small bowel (ileum), large bowel (colon) and even in the anus. Colitis, another IBD, only affects the colon, and only the inner lining, whereas in Crohn's Disease all of the tissue of the bowel is involved.
Crohn's symptoms include abdominal pain, nausea, vomiting, constipation interspersed with diarrhea (that can be bloody) and weight loss. There are also symptoms that aren't bowel related like skin rashes, joint pain and not as common, inflammation of the eye.
There are still some that are unsure whether or not Crohn's is hereditary, but most agree that it is. There is also much debate about what it is caused by, but again most do agree on one thing. There is a dis-connect between the immune system, the brain and the digestive tract. There is a protein in the body that is called "Tumor Necrosis Factor" (TNF), and a larger quantity of this protein is said to be found in Crohn's patients.
One huge misconception about Crohn's Disease is that all patients are created equally. This is not true at all. We are all as different as night and day. We all have different symptoms, can or cannot tolerate different foods and our disease affects us each uniquely. Some patients don't ever require surgery to have diseased bowel removed while others may have need many.