Steampunk generally starts from a premise of what would have happened if the world of Jules Verne had become reality, or to put it another way, if the internal combustion engine hadn't been discovered, how would the world look now. Also consider books like Consider Phlebas by Iain M Banks, or movies like the Golden Compass or Stardust (love that film).
Cyberpunk generally starts from the world of today, and pushes it forward to consider worlds where virtual reality is a place you can go for days at a time. Places that are described by comics like 2000 A.D. (think 'Judge Dredd'), shows like Almost Human, or films like Blade Runner and The Matrix (one of my all-time favourite movies).
Cyberpunk tends towards the dystopian, where Steampunk tends to be more refined and cultured. Though that's a very broad generalisation as there are strong exceptions in both cases.
ETA: if you Google 'steampunk movies', you'll see the recent Robert Downey Jr/Jude Law Sherlock Holmes movies get included. These are not Steampunk, because they're set in late Victorian/Edwardian London (around the turn of the 20th century), and whilst the image of London shown is a Hollywood pastiche, it is roughly contemporaneously right-ish - the technology is right (-ish), and there are cars.
Last edited by gremlin; 03/11/17 04:20 AM. Reason: rant about movie mis-categorisation