Neti is an important part of Shatkarma (sometimes known as Shatkriya), the yogic system of body cleansing techniques. It is intended mainly to the cleaning of the air passageways in the head. Both the Hatha Yoga Pradipika and other sources usually attribute to Neti many beneficial effects that range from profound physiological ones on the body, mind and personality to even clairvoyance. The two main variants are jala neti using water and the more advanced sutra neti using string.
Definition courtesy Wikipedia.