A term that has been coined by "purist" dubstep fans to distinguish their preferred genre of music from anything mainstream. Like many hipster-based opinions, the only real classification that can be made to differentiate brostep from dubstep is that brostep is more popular.
The two forms of music both focus on sweeps through very low frequencies of bass and what many like to call "wub" or "womp". Or to the rest of us "What the hell is that noise? How is it at all enjoyable? You can't even dance to it!"
A remix of Cinema that was recorded by Skrillex is said to be "Brostep", but only because it thrust the genre into popularity after being used by the WWE and receiving a grammy.