Short for Dominance / Submission, Dom/Sub is the primary way gay men who are part of the dominant culture of anal penetration conceptualize their sexuality and their relationships, and is a direct consequence of the central place of anal penetration in the culture.
Just as anal penetration, in its insertive aspect, appears to mimic penile-vaginal sex, so does Dom/Sub appear to mimic the masculine and feminine roles of heterosexuality.
Thus synonyms for Dom/Sub include top/bottom, butch/femme, butch/bitch, master/slave, and daddy/boy, and most men self-define as either a top or bottom, though some say they are "versatile" -- able and willing to play either role.
The degree to which the Dom/Sub roles permeate gay male culture can not be overstated. The most common question at a gay male social gathering is "Are you a top or a bottom?" And two men in a relationship will routinely be asked "Who's the top and who's the bottom?"
Men who are part of analist culture have difficulty understanding that there are no tops or bottoms in the practice of Frot, and have trouble conceptualizing a relationship built on Frot, since there's no clear division of roles.
The idea that two men can be equal and equally masculine in a relationship is, for men whose norms are those of analist culture, genuinely perplexing.
See also effeminacy, heterosexualization.