I get this question a lot, especially when I tell people I sing in a rockabilly band (Sidecar Steph & the 7-10 Split). Rockabilly can be so many things to so many people, so I'll keep my definition to what I think it is and perhaps others will add on through their comments. I could wax poetically for hours on end on this subject, but I'll save that for my masters thesis and keep this post as simple as I can.
Rockabilly as a word is "rock+hillbilly" and many of the early musicians were considered country & western, like Rose Maddox & the Maddox Brothers and Charline Arthur. There were also quite a few jazz musicians that would contract in to a studio session, like Les Paul, and Sun Records was crawling with players from both genres. Sun Records is considered by most to be the birthplace of rockabilly music and they owe much of that to Elvis Presley. Here's where it gets interesting- Elvis was heavily influenced by African American music, specifically gospel and R&B bop, and this, coupled with the aforementioned genres, provided the driving beat that really gave rockabilly it's unique sound. So now we have three diverse influences helping to create this new and exciting sound.
When people think rockabilly they usually remember the men of the genre, like Elvis, Johnny Cash, Jerry Lee Lewis, Buddy Holly, etc. However, there was a huge female contingency that could rock just as hard as the men- Wanda Jackson (pic on right), Janis Martin, LaVern Baker and Barbara Pittman are just a few.
Rockabilly music unfortunately went as quickly as it came, mostly because of social issues and a fear of "the beat, the beat, the beat." Luckily, interest was revised in a big way in the 1980s by groups like The Stray Cats and The Blasters. Psychobilly (punk+rockabilly) also came out of this movement led by the group The Cramps. The genre just continues to grow and thrive, with it creating an attractive lifestyle that goes beyond the music.