Theme and variations is a musical form in which a melodic idea (a melody) is stated (played) and then varied (changed a little) in a succession of statements (and many of these slightly changed versions are played, one right after the other).
Here is the original song called, "Johnny I Hardly Knew Ya" which dates back to the early 1800's.
"Johnny, I Hardly Knew Ya"
As Irish immigrants fled the great potato famine in the 1849's and moved to America, they brought this song with them. 15 years later during the Civil War, many of these men who had come to the states from Ireland ended up fighting in the American Civil War. The song became adapted to what we know as, "When Johnny Comes Marching Home Again."
"When Johnny Comes Marching Home"http://www.youtube.com/watch?v=7v2mW21M0cw
"When Johnny Comes Marching Home Again" was revamped for orchestra by Martin Gould in 1943 and titled "American Salute". It was very popular and Martin Gould transcribed it for band. Martin Gould's version is a classic example of theme and variations.
Finally, the modern group, the Dropkick Murphy's performed a rock version of the original, "Johnny, I Hardly Knew Ya"