A musical theme is a melody that's introduced at the beginning of a piece and repeated over and over again throughout the rest of the song with slight variations. It's quite simple, really, because only one tune is really involved, but it sounds great. Pachelbel's Canon in D is a wonderful example of a theme and its variations. This is not to say that these types of music were complete ramblings of notes - think of Queen's Bohemian Rhapsody , a song that's wacky and meandering, but you can still sing and violently bob your head along with it because it has a distinct tune and rhythm. Tone Poem .
Tone poems are similar to rhapsodies in that they have no specified structure.
But their specialty is to tell a story. A great example of a tone poem is Sergei Prokofiev's Peter and the Wolf . Or, for a more modern example, John William's Star Wars score. Nowadays, overtures are composed to be played before the curtain goes up at a Broadway show or opera. Some classical music was composed for the explicit reason of accompanying the ballet.