"Bro" is a term for a style or subculture of young men, and like many terms for subcultures, the denotation and connotation of the term can be somewhat hard to pin down. Perhaps part of this is due to my age: at 33 I must admit that I am finally removed from having an intuitive grasp of what subcultures people are in. And indeed, I know that the depth of subcultural affiliation, so important in adolescence, will soon vanish sometime around the 25th birthday when people have to start worrying about dentists and unpaid bills. But I digress.
"Bro" refers to the way these young men hail each other, a casual form of "Brother". As such, the uninitiated might think that "Bros" are fraternity brothers, and while they bear some relation, they are not the same thing. The opinion on this seems to be split: the plethora of definitions on Urban Dictionary seems to cover the gamut of interpretations of "Bros", from wealthy fraternity brothers to skate punks. Some of them are rich, some of them are poor. Their hobbies include beer, weed, cars, extreme sports and gaming. Their attitude might be described as one of casual insensitivity. The problem with using these as descriptions of what a "Bro" is like is that these are pretty much the way to describe the average man between 16 and 25.
As a subcultural label, "Bro" probably fits the "average" guy of late adolescence, who has not aggressively claimed another subculture. Along with that, the "Bro" casually adopts to many things that might have, ten or twenty years ago, been reserved for a different subculture. A Bro might go skate boarding, then come home to watch Star Trek and play some games while smoking a bong--- things that might have once been reserved for the skate punk, geek, or hippie/stoner subculture. In other words, in the fast shifting world of popular culture, the Bro currently occupies a middle ground where once countercultural activities have become mainstream.