What is Americana?:
Americana artists frequently pull from traditional and contemporary folk, bluegrass, country, alt-country, soul, gospel, and rock. While this may sound a bit vague or confluent, Americana artists and fans are content to say they know it when they hear it. The best way to learn what defines Americana music is to listen to it (and debate at length with other fans).
Americana Artists:
Some of the most popular and influential Americana artists include
Lucinda Williams,
Gillian Welch,
Old Crow Medicine Show, Alejandro Escovedo, Buddy and Julie Miller, Eilen Jewell, Ryan Adams, Drive-By Truckers,
Emmylou Harris, Todd Snider, Peter Case,
Johnny Cash, JJ Cale, and many more.
Americana Instruments of Choice:
Americana musicians, like those in so many other areas of folk music, tend to build their bands around acoustic guitars. However, considering the wide range of styles amalgamated by Americana artists, it's not rare to see a full rock line-up and the use of other instruments like piano, melodica, vibraphones, and other instruments.
Recommended Americana CDs :
Lucinda Williams -
Happy Woman Blues (Smithsonian Folkways)
Gillian Welch - Time (The Revelator) (Acony)
Johnny Cash - America (USA)
Avett Brothers - Emotionalism (Ramseur Records) 
Americana Music Labels:
Several indie labels are reliably releasing great Americana music these days, including Red House Records,
Ramseur Records,
Rounder Records, Anti- Records, Bloodshot Records, Nettwerk, Signature Sounds, and many others.
History of Americana Music:
Americana music has been loosely defined through the years to include the fundamentals of American roots music based in classic-style country, but including all the influences that converged to give birth to early rock and roll. It's emerged more definitively over the past several years, through the organization of the
Americana Music Association and its influence in the industry, but the very definition of what constitutes "Americana" is still mostly up for interpretation.
One thing is certain, though. More and more artists are richly influenced by a wide range of quintessentially American musical styles—from R&B and soul to classic country and modern rock—and are, through making their own music, slowly defining and evolving the Americana genre.
The genre began as a radio format in the 1990s, which wasn't the beginning of the style of music but was rather a benchmark in the definition and discussion of the style—a discussion which has been kept vibrant by the AMA and publications like No Depression (now NoDepression.com) and various blogs such as AmericanaRoots.com, TwangNation.com, The9513.com. With a decade under its belt, the Americana Music Association continues to hold an annual conference and festival in Nashville to discuss the evolution of Americana music and present awards to its members.