Smegma is the greasy white stuff which is found between the boy's glans (bell end) of his penis and his foreskin. It is made of natural lubricants mixed with the dead cells that quite normally come off the glans of your penis (the skin of all parts of your body renews itself regularly by shedding dead skin cells and growing new ones and the end of boy's penises is no different). Smegma can be a bit smelly (knob cheese) especially if you don't regularly wash it away. Women can get it in their vaginas and vulvas as well. It acts as lubricant when making love, and is less common in men who are circumcised.