Prosumer is a neologism first coined by Alvin Toffler in his 1980 futurist book The Third Wave as a portmanteau of dual origins: both (1) professional and consumer and (2) producer and consumer.
In the first sense, a prosumer is a highly trained consumer of professional equipment—a midway point between someone with an entry-level skill set and that of the professional. In the second sense, a prosumer is a consumer who buys equipment with which to make new media. The distinction is subtle enough to almost be immaterial, but hinges upon the idea of a professional chasing large media conglomerates for contracts versus that of a self-published amateur.
Initially, only the first meaning of prosumer was common (and even then, its use was limited) in relation to serious audiophiles and to "mid-range" dSLR cameras. However, a true practitioner of those arts of sound and vision will be quick to tell you that the label "prosumer" can be deceiving as it is not the tools one employs but the talent and sense of balance which can only come through dedicated practice. Use of the word in the secondary sense has expanded exponentially with the advent of the internet's ubiquity to include people using cutting edge technology to create and share new media online for consumption by the teeming masses.
While the word prosumer has not yet risen above the status of jargon, the scope included in this secondary sense outnumbers that of the first.
Some pedants hold that the correct term for this latter phenomenon should be "pubsumer" as many of the participants in this internet "revolution" are amateurs at best
and merely publishing outright crap. But that term could be confusing for other connotations of that prefix
The "internet revolution" as it pertains to commerce has further expanded the idea of the prosumer to include individuals who take an active role as a customer, engaging in research in an effort to better the overall quality of goods available in the market. This is a crowdsourced effort accomplished by blogging, writing reviews and cataloguing products, and giving a company instant feedback viewable by anyone—investors, analysts, and other potential customers.
But, like much of the Web 2.0 hype, this is not really anything new. An argument could be made that proto-prosumers forced Ford to offer automobiles in more than one color. This was done through newspapers (which once printed multiple daily editions) as well as word-of-mouth.
Had there been a vibrant prosumer culture in the 1980's, Betamax may not have lost to VHS. But maybe that's a bad example, as that particular case likely has more to do with the contracts of certain programming being released primarily on the inferior format.
More recently, the word "prosumer" has been applied to those in the "maker" subculture: folks with either easy access to a 3d printer or who would otherwise fall into the DIY category—maybe they make their own condiments or bookshelves or quilts, often at a cost (in terms of time and money) which is greater than purchasing a mass-produced version of said item.
In this sense, the word is more simply understood as being directly opposite of consume.
Prosumer is also the stage name of the German DJ and producer Achim Brandenburg, who does progressive / deep house. He's really good, if you're into that kind of thing.
Here's a short documentary in which Prosumer discusses his approach to music.