The jejunum is the part of the intestine between the duodenum (the first section of the small intestine) and the ileum (the third and last section of the small intestine).
The major difference between the jejunum and the ileum is the jejunum is the second, or middle section of the small intestine and the ileum is the third and last section of the small intestine. In function, the jejunum
has a lining which is specialized in the absorption of carbohydrates and proteins.
The ileum, on the other hand, is the section of the small intestine that absorbs whatever nutrients the first and second sections didn't get, as well as Vitamin B12 and bile salts. It contains the greatest number of villi (tiny, finger-like projections) per square inch to maximize final absorption. The ileum works as the "finisher" to the absorption process before the digestive mass empties into the colon. The ileum also has more fat cells inside its lining (mesentery) and is loaded with lymphatic tissue to aid the immune system.