This discussion takes place over five pages (starting with this one), with links at the bottom of each page to carry you to the next one. First, we need to meet the person who got the ball rolling in terms of thinking about innovation and diffusion...
Someone to know: Everett Rogers
Before we even begin this topic, we need to meet the defining figure of the field: Everett Rogers.
Rogers was a professor at Ohio State and did his seminal research at Iowa State during the 1950s, figuring out why some farmers would adopt an innovation (e.g., a new pesticide or a new hybrid seed) while others wouldn't. This topic was (and is!) of interest to the agribusinesses (e.g., Archer-Daniels-Midland, Monsanto) but also the US government, which was interested in boosting agricultural productivity.
His major contribution, The Diffusion of Innovations, is supposedly the second-most cited book in the social sciences (per Wikipedia, sourced to an out of date link...so don't take that as gospel).
OK, so how does research on 1950s Iowa farmers become wildly relevant to so much other stuff??
Let's start with some terminology.