60 second music lessons by Dan, Vol 1:
Dub predates dubstep and the ilk by almost half a century, it's origins stem from the producer Lee Perry, who recorded a ton of legendary reggae acts of the time, including marley. Dub grew out of reggae. In the most simplified explanation, dub can be described as reggae instrumentals with boosted lows. the dubplate (completely different use of the word dub, but it actually ties into the genres developement) grew in popularity with dub culture, as it was the easiest way to get remixes out to DJs before the internet existed.
Dubstep got it's origins from drum and bass, itself a successor of Dub. D&B grew into its own took dub and doubled its speed. Dubstep took that sound and halved it again, but placed even more emphasis on the lows and with the addition of synthesized basslines.