Male members of a 'social' group on a college campus.
Males who participate in close concert with other like minded individuals on more of a social scale, that is more defined, ritualistic.
Generally a North American ritual, based loosely on the old German or Prussian collegiate action of forming clubs based on class structure. Many involved dueling, while today's fraternities are more about socializing.
A highly subjective term, simply referring to some male member of a local fraternity. In sexual terms the 'fraternal' association revolves around sexual activities, and can include various other elements of sexual niches.
This can include spanking, paddling, ritualistic garb or clothing, done to highlight the association or bonding between members. Sexual sites also tend to equate 'frat boys' with males between the ages of 18 and 25.
In the 'gay culture' the use of the term 'frat boys' is more of denoting an age group, involved in a social setting where anything goes. This can include BDSM rituals, but in a minor form than BDSM. In addition is used to denote a genre or class of men, who are generally more well built physically, than ordinary 'twinks' might be.
Smooth bodies, a criteria generally associated with 'twinks' is not a requirement for 'frat boys'. The more closely aligned category to 'frat boys' would be 'jock' rather than 'twink'.
The term is more associated with wild parties, with wild multiple sexual encounters. The term also can indicate the depiction of more multiple sex partners in a more public venue, such as in a condo or home. It has the implication of orgies, rather than one on one sex, though that is also depicted.