Perhaps the most famous brand, KY Jelly is a personal lubricant used for sexual activity. It is used during sexual intercourse to provide extra vaginal lubrication. It and other types of lubricants are required for anal sex, since the rectum does not provide its own lubrication. Since it is a water-based lubricant, it is also ideal for any sex toy.
KY Jelly is also useful outside of sexual activity. Latex clothing, while fashionable and ideal as costuming for BDSM activities can be difficult to get into without damaging the garment. Lubricants like KY Jelly can be applied to the body so that the latex clothing slips of easily without tearing. Since it is water based, it will not stain or otherwise damage the latex material and excess wipes off the skin and clothing easily.