Brostep is dubstep that has a lot more high pitched and "filthy" sounding basses in it. What most people call dubstep focuses on slower, lower basslines. I think a good example would be to listen to excisions' 2008 shambhala mix, which is a good example of dubstep, then listen to the 2010 shambhala mix, which has a lot of brostep elements in it. Just listen to the first 5 minutes of each mix and I think you'll be able to see the difference.
skrillex is brostep dubstep that panders to the lowest common denominator is brostep, ergo all popular dubstep is brostep. A bunch of suburban white kids trying to be "filthy" without actually knowing anything. I made a dubstep track once and someone called it brostep, I dont disagree though because I dont go around usually making dubstep or consider myself even able to make "real" dubstep. furthermore; dubstep. Dubstep, dubstep- dubstep, dub,step, dubstep? dubstep.