Snowballing is a humansexual practice in which one person takes into his or her mouth the semen of another person, and then 'swaps' it back and forth with another person through kissing or spitting.
In a heterosexual context, snowballing may appeal to a male who wishes to adopt a passive or submissive role (which conventional fellatio does not entail). Alternatively, a couple may simply find pleasure in the sharing of an experience. Most people, however, seem disgusted by the notion.
Snowballing often features in pornography, sometimes as part of a practice called bukkake. Here, men generally adopt the role, and semen is passed between women's mouths.
Because snowballing involves the exchange of bodily fluids without any barrier, it is not a form of safer sex. Risks are presumably similar to those of unprotected fellatio.
External links
Dan Savage describes snowballing in his Savage Love advice column (http://www.thestranger.com/2003-04-24/savage.html)