According to Wikipedia:
Rockabilly is one of the earliest styles of rock and roll music, dating to early 1950s in the United States. It is a “blend of country & western and rhythm & blues” that… pointed the way to classic rock ‘n’ roll.” It has also been defined as “popular music combining features of rock ‘n’ roll and bluegrass.” 
The term “rockabilly” is a portmanteau of “rock” (from “rock ‘n’ roll”) and “hillbilly“, the latter a reference to the country music (often called “hillbilly music” in the 1940s and ’50s) that contributed strongly to the style’s development. Other important influences on rockabilly include western swing, boogie woogie, and rhythm and blues. While there are notable exceptions, its origins lie primarily in the Southern United States.
The influence and popularity of the style waned in the 1960s, but during the late 1970s and early ’80s (notably with the Stray Cats), rockabilly enjoyed a major revival of popularity. An interest in rockabilly endures even in the 21st century, often within a rockabilly subculture.
Well ladies and gents, the term “Rockabilly” is synonymous with the rock ‘n roll style music of the 1950’s. Think Rock-meets-Country music. Elvis is a great example of Rockabilly style music. Rockabilly is just another term for a sub-culture such as “punk” or “scene” where people enjoy a certain genres of music, style of clothing and hair, and events.
Now that we have the history lesson our of the way, lets move on to What Is Rockabilly Clothing…
Rockabilly is a term used to refer to anything retro. Rockabilly clothing usually refers to items such as; Cute polka dot retro dresses, dresses with dainty cherry prints, full circle skirts, vintage items from the 1950’s, and anything else that has that 1950’s appeal. “Rockabilly” does not just refer to a style of clothing, but more of a “lifestyle.”
Modern day male rockabillies often wear slim jeans, have a bandanna somewhere (ie. back pocket, or worn as a wrist cuff), and often wear a retro or vintage shirt, while female rockabillies love any sort of retro dresses, skirts, and tops, and wear dolled up hair. One of the more popular rockabilly hair styles among the ladies is the “Bettie Hair.” This refers to hairstyles similar to that of Bettie Page. Retro hair accessories are a must have for anyone trying to achieve the rockabilly look. Both male and female rockabillies today may adorn themselves in vibrant tattoos, and piercings, but don’t confuse them with “Psychobillies.”
“Psychobilly” is very similar to “Rockabilly” but usually has darker undertones in both music and dress. Some great physchobilly bands are Tiger Army, The Creepshow, and The Horrorpops. The clothing may consist of retro inspired clothing, but with darker or morbid prints, such as zombies, skulls, etc…instead of polka dots and cherries.