In 2002 the magazine XLR8R put out an issue about the emergence of dubstep. I still have the copy that I bought on the stands, having never heard of dubstep.
Here the magazine writes that a BBC Radio 1 DJ credits XLR8R with coining the term, although the writer of the article is not sure if that's correct.
Anyone paying attention to electronic music since the early 00s can easily tell the difference(s) among dubstep that has been made since then. Like Freebase said, it's not a linear progression of a certain sound.
In that issue of XLR8R many unknown DJs are written about. One duo that definitely helped start the 'genre' (and one of my personal favorites) is Horsepower Productions. I consider them 'old' dubstep, although they don't represent the entire early scene which I think stems from a pretty simple equation: UK garage + Dub + 2-Step. But even that simple combo yields extremely varied results.
Here's a song HP made in 2004
And one in 2009
'New' dubstep is less easy to define because there are so many styles incorporated into the music that gets that tag. I think what Freebase called 'brostep' is generally what people think of when they hear the word now. Like this Zomboy stuff from, I think, 2013 (I personally think it sounds awful)
Of course there's a lot more than that coming out currently. Tempa is a label that has released reliably good stuff both old and new.