Bariatric surgery is indicated for those who are 'morbidly obese' and in whom conventional modes of weight-loss treatment have failed.  These individuals may be also be at increased risk or already suffer from severe diabetes or cardiovascular disease.  Individuals with "severe obesity" typically have a BMI of 40 or more. 

 Basically, the proceedure involves limiting the size of the normal stomach, so that even small quantities of food produces a sense of fullness and eventually leads to weight-loss.  These involve proceedures like gastric banding, gastric bypass, and vertical-banded gastroplasty. 

Bariatric surgery is a 'last-ditch' effort and carries with it the complications of any major surgery.  Studies indicate a survival advantage after undergoing such proceedures.