Louis Armstrong performs in Paris in 1965.

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­Dixieland, swing, bebop, acid jazz, hard bop and fusion -- these are only a few of the variations of jazz that have developed over the past 100 years. They can sound drastically different from one another, yet we call them all "jazz." Why? What is it that makes jazz "jazz"?

­­­­Jazz -- commonly heralded as "America's Classical Music" -- defies an easy definition. And, according to Louis Armstrong, one of jazz's greatest performers, if you have to ask for a definition you'll never know what it is. While there is an element of truth to Armstrong's you-know-it-when-you-hear-it philosophy, critics and historians have sought to understand and describe what makes jazz one of the most exciting, unique and complex forms of American music.

Jazz was created in New Orleans around the turn of the 20th century and is best understood in terms of its basic elements: improvisation, syncopation, rhythm, blue notes, melody (the tune) and harmony (notes that fit well with the tune). The music really started taking hold of the United States and the world during the Jazz Age of the 1920s, when advances in radio and recording technology allowed millions to embrace the jazz culture.

­­In this article, we'll take a look at the most important elements of jazz, how to listen critically and learn about the music's rich history.