The transition from old media to new media can be likened unto the days of Gutenberg. One day this guy, who we’ll call Johannes, decided that it would be really trippy if books could be made faster than a handful a year. So he went off and made a printing press, and all of a sudden books were flying out the wazoo (comparatively speaking, anyway). If you could access the technology and knew what you were doing, you could print whatever you wanted for everyone who could read to see. Oh yeah, and this nifty little invention led to a lot of people learning how to read.
Skip ahead a few hundred years: society has settled into a producer-consumer relationship: a select few made stuff, and the rest of society consumed it because there wasn’t anything else. Their opinions on the content didn’t really matter, either, because they neither had the power nor the ability to create alternatives. So they were stuck with what the media aristocracy gave them. Kind of like the people of Gutenberg’s day. And then came the internet, and, once again, if access was present, things could be made by anyone. And I mean anyone. Ability was, and is, irrelevant; if you can get it there, it’ll be there for everyone to see (peruse deviantArt or Tumblr for five minutes if you still believe ability has anything to do with content creation).
While this would suffice as a definition for new media, there’s also the small detail of audience interaction. With old media (the stuff from the old-timey the producer-consumer relationship), there is a monologue you’re forced to sit through: their spiel. You get no say or input. It’s like Hamlet, and you’re Polonius behind the curtain. Speak against them or try to interact, and next thing you know, “I am slain!”
However, with new media, you can interact however you want, whether that be commenting, sharing, creating fan art without legal “retribution,” or even joining in the creative process. Some people refer to this as the “death of the author,” and, in many instances, it is. However, in others, it is the birth of the “collective creator.”