Most true punks like to simplify cyberpunk into one term (much like steampunk) "high tech, low class". We see this a lot in the crazy cyberpunk 80s movies that have recently been making comebacks: Bladerunner, Demolition Man, Total Recall, Tron, RoboCop and all the rest. There are also modern movies that are clearly cyberpunk like Surrogates and Babylon A.D. But then you have the more modern movies that some people are pretty skiddish to call cyberpunk, but clearly fit into the definition. These movies of course include Minority Report, Paycheck, and Avatar.
Where do you draw the line between cyberpunk and straight up sci-fi? Where do you draw the line between cyberpunk and post-apocalyptic? In my case, where do you draw the line between steampunk and cyberpunk? For me these fall under the same category as questions like "where do you draw the line between sci-fi and fantasy?". The answer is that you can't clearly draw a line, which is why book stores put them all in the same jumbled up section under the label of "Sci-fi/Fantasy" and call it a day. In my opinion, it's up to the authors to decide, not the reader.
For me, I feel as though I am clearly in the realm of steampunk even though any hard core hyper-classifier would consider me to technically be an atompunk because of the era it takes place in. (You can find the list of sub-genres here.) I mean, the difference between atom punk and steampunk are simply explained with these pictures:
Then again, if you look very closely at my world, it could be clearly argued that I write a retro form of cyberpunk that isn't really classified as steampunk at all! In fact, with CeLeSTe being an AI that pretty much runs the ship along the same lines as Cortana from Halo, and with all the talk of advanced biology, reprograming brains, pockets of space-time, and glass interfaces, it begins to sound very cyberpunk. To all of these arguments I say "Shut your pie hole and go write your own story" and I wish to inform you that while atom punk is pretty darn cool, it wasn't the esthetic I was going for.
For those of you still confused by all my uses of the term "punk" and the argument in general, I have to say I'm right there with you. Here's the long and short of it. Cyberpunk has evolved over the years to include a number of different sub-genres including but not limited to!: Steampunk, Teslapunk, Dieselpunk, Decopunk, Atompunk, Biopunk, Nanopunk, Stonepunk, Nowpunk, Splatterpunk, Elfpunk, Mythpunk, and now including Rococopunk (yeah... look it up - it's pretty much along the lines of the latest version of The Three Musketeers). These are all technically subcategories of Cyberpunk, namely they are featuring some sort of "high _____" (whether it be biotech, magic, or steam) and definitely includes "low class".
Notice that all these sub-punks have one thing in common. Yeah, you guessed it. "Punk". These genres are for the rebels, the crazies, the teens, and the working class. This is the major thing that sets any of these genres apart from historical fiction or literary fiction or straight up sci-fi/fantasy (which is a point of great contention among fans and creators in these fields).
So, if you're looking for a straight up clean cut definition of what the difference between a steampunk and a cyberpunk is, you aren't going to really find one other than "they obviously take place in different time periods".
That said, I want to hear your takes on what sets these sub-punks apart or what you would classify The Shadow Cast Chronicles as in the comments below!
You will also find in each of these sub-punks there is a heavy focus on fashion and gadgets. Seriously! Look them up! The first thing that pops up in each search is "____-punk Fashion".