Abdominal Surgery includes anything within the abdominal cavity. This includes the liver, spleen, gall bladder, stomach, small bowel, large bowel, appendix, pancreas, and even the blood vessels to the bowels.
The most common surgery is the gall bladder removal. The next would be an appendix removal. Colon resections and small bowel resections (removal of segments) are also done, whether for cancer or injuries, or other reasons. Sometimes exploratory surgeries are performed where the structures are evaluated for abnormalites, usually looking for a source for pain.
Many of these surgeries can now be completed with a laparoscope. Laparoscopic surgeries are basically the same surgery with less trauma to you, as the patient. This means quicker return to normal activities.