Polka - The Basics:
Polka music is a form of European dance music which originated in Bohemia (what is now an area within the Czech Republic). It migrated to the United States with Eastern European immigrants and remains quite popular in areas of the Midwest and Great Lakes Region. A polka song is often just referred to as "a polka", and polkas have found their place in both the folk and classical repertoires.
Polka is often associated with the German Oktoberfest, but in reality, it's much more popular in Czech and Slovakian regions (the music that you hear during Oktoberfest is related, but not the same). European polka is slightly "straighter" and more traditional than American styles, having fewer outside influences.
Polka in the United States:
There are discernible differences between the polka styles of, say, South Texas and Cleveland. These differences mostly depend on the ethnic influences of the separate regions - in a region with more German immigrants, the sound became more oompah-influenced. In a region with more Mexicans, the sound became much more Latin.
The Polka Beat:
Traditionally, a polka is a dance in 2/4 time. Many other genres of music have the occasional polka in their repertoire, including Cajun Music and old-time. However, polka bands also often include other song styles in their repertoire, especially the ever-popular waltz.
The Polka Sound:
For most people, polka is linked at the hip with the accordion, and indeed, it's the force behind every polka band. Polka bands also, depending on their region, often include fiddles, clarinets and a rhythm section. The 2/4 rhythm of the basic polka has a very bouncy, upbeat sound - great for dancing!
Polka in the Classical Repertoire:
Many Eastern and Central European composers of the Romantic Period composed polkas, especially the Strausses. These 2/4 timed masterpieces are still being performed today, keeping the link between folk and classical music alive.