Senna is an herb. The leaves and the fruit of the plant are used to make medicine.
Senna fruit seems to be gentler than senna leaf. This has led the American Herbal Products Association (AHPA) to warn against long-term use of senna leaf, but not senna fruit. The AHPA recommends that senna leaf products be labeled, "Do not use this product if you have abdominal pain or diarrhea. Consult a healthcare provider prior to use if you are pregnant or nursing. Discontinue use in the event of diarrhea or watery stools. Do not exceed recommended dose. Not for long-term use.”