One of the classes of customers listed in CrossingTheChasm. The others are Innovators (self-explanatory), EarlyMajority?, LateMajority? and Laggards.
Early adopters are those people who pay too much for something that doesn't quite work yet. They buy new, unproven products/technologies. They put up with all the production delays, bugs, defects, missing features, cryptic documentation, lack of third-party support, and risk that the product will never work right. They do this because (a) new stuff is cool, or (b) they think early adoption will lead to an advantage over competitors who are more conservative.
Early adopters are the people who bought CD players back when only a few dozen titles were available, or who paid $3000 for an original 128K Macintosh, or who embarked on an enterprise software development effort using Java Development Kit 1.0.
Without early adopters, technology-based industries would falter. However, companies that depend upon early adopters take a big risk, because if their products don't catch on as "cool", the products won't sell and so they won't be able to generate income to improve the product for more widespread acceptance beyond that of the techno-nerds. There is also the risk that a competitor can lure potential early adopters away with announcements of similar products (see VaporWare).
See CrossingTheChasm for more information and references.