In a nutshell, dubstep's originally developed roughly around the turn of the century in the UK with a lot of heavy influence from Drum n Bass, UK Garage, and various sounds that were heavily influenced by dub reggae. What this roughly translated into sonically was a rhythm structure similar to dub reggae with drum flourishes similar to two-step garage, with bass drops similar to that of Drum n Bass.
Example that I always enjoy using:
Skream - Lightning
http://www.youtube.com/watch?v=KnMFIeDhT5Y
In this track a lot of the genre's influences are on display: the slow rhythm structure of dub reggae, usage of quick-edit drum samples similar to that of UK garage, and the bass drop as well as the ACTUAL Amen Brother drum sample that practically defined Drum n Bass.
Somewhere down the line, an event that's common in electronic music occurred: someone took one or two elements of the genre to its logical conclusion, making it the sole emphasis of the track and cranking the levels and repetition to 11. In this case, what this was done to was the buildup-bass drop structure, as well as the over emphasis on what were generally dark and rumbling basslines. The person that many people blame for this btw, is probably Rusko, a producer from the UK.
This resulted in the branching of the genre. You had now a faction that continued the route that Rusko took. The emphasis on the aggressive basslines actually was first used somewhat symphonically by artists such as Nero, Caspa, Chase & Status, etc. However, around the same time there were producers taking what Rusko started and took it to its logical conclusion, using more abrasive and distorted sounding basslines to further driving the aggressive tone of the track. This is the branch that leads to what people refer to as Brostep.
In the meantime, purist dubstep still existed. The old producers didn't really go anywhere. You had the pop crossover of Magnetic Man (which consists of old genre stalwarts Skream, Benga, and some dude named Artwork), you had guys like Kode 9 and the whole hyperdub label doing more experimental tracks within the genre. There were even people that even straight up veered into downtempo while keeping to the general roots of the genre; Phaeleh is one such producer that does this off of the top of my head. Something that I think confuses people is that quite a few of the old hands still dab in the more aggressive vein of dubstep.
Something that has also emerged now is what I and some others refer to as the "Post Dub" sound. This is essentially club friendly house/dubstep that incorporates a lot of freeform influence from everything including but not exclusive UK Garage, UK Bass, IDM, classic house, hip hop, and soul/funk. It's incredibly difficult to define because the sounds involved are so varied, but it's also rather easy to enjoy. Producers in this genre off of the top of my head include Jacques Greene, Joy Orbison, Sbtrkt, Falty DL, Hackman, Koreless, etc. Some people are still trying to do dubstep with a relatively purist view on the rhythm and structure, but incorporate new sounds to back it up, like Rustie and Hudson Mohawke.
Oh yeah, I didn't mention the Burial and Four Tets of the world for a reason: their had next to nothing to do with dubstep and people just call it that because their work on hyperdub got popular and it was a convenient association.
If you don't know now, you never will.
