Theme and Variations
What is it?
A theme is a musical idea or a melody. When a composer changes the melody in different ways but you can still recognize that the melody is there - that's a variation. Kind of like the picture above. The purple smiley face is changed but you can still recognize that it's a purple smiley face. Here are two really clear examples of theme and varation.
Famous Variations
Many composers have written variations. Sometimes a modern composer will write variations on a theme by an earlier composer. That's what Benjamin Britten did in "A Young Person's Guide to the Orchestra" which you can watch below. (If you want to learn more about this piece and the instruments of the orchestra you can go to this online game by Carnegie Hall Listening Adventures.
Sometimes a composer will use a well known melody as the theme. Mozart did this with his 12 Variations on Twinkle Twinkle Little Star. Composers can also write their own theme and then write variations. The theme in the Haydn piece below even became the Austrian National Anthem. Sometimes a composer will use theme and variation form in a movement of a symphony or sonata (similar to a symphony but for one instrument). Beethoven does this in both the Seventh Symphony and the Ninth Symphony.