Chorales tend to have quite simple and easy to sing tunes. They generally have rhyming words and are in a strophic form (with the same melody being used for different verses). Some chorale melodies were written by Martin Luther himself.
Although originally sung in church a capella, several composers arranged and harmonised the melodies for several voices. Johann Sebastian Bach harmonised many chorale themes for a four part choir comprised of sopranos, altoss, tenors and basses. These were used as congregational hymns interspersed in his cantatas, passions and other works. These harmonisations are so well known that Bach's name is virtually synonymous with the chorale in classical music circles, even though he did not write any original chorale themes himself. Bach also frequently wove chorale melodies into larger choral works as a counterpoint to other themes.
Another use of chorale tunes in classical music is in the chorale prelude, a piece generally for organ designed to be played before a chorale, as an introduction. They include the melody of the chorale, sometimes making it the theme as a set of variations, sometimes subjecting it to contrapuntal techniques. One of the first composers to write chorale preludes was Dietrich Buxtehude. Bach also wrote many chorale preludes, which are probably the best known examples of the form. Later composers to write them include Johannes Brahms and Max Reger.