Senna is a 3-foot tall shrub that primarily grows in North Africa. Its small leaves and pods are used medicinally for laxative purposes. It's such an effective "cleansing herb" that it sometimes shows up as an ingredient in weight-loss products, but it's not a very pleasant way to lose a few pounds, and certainly not long term solution.
The plant contains substances called "sennosides" that act as an irritant to the intestinal lining and trigger the bowels to evacuate. Sometimes this is accompanied by nausea, pain and/or diarrhea, so senna is often combined with calming herbs like fennel or ginger to help soften the blow, so to speak.
Beware, senna is such a strong purgative agent that it should only be used in cases of severe constipation or if you need to expel parasites from your GI tract. Do not use senna if you have any inflammation within your digestive system, and never exceed the recommended dosage.
Senna can also be used as a topical agent on the skin to treat acne or ringworm.