A slightly more complex form is a verse-chorus arrangement where one section of music returns with new words each time--the verse--while a contrasting section returns with the same words. This section, called the chorus because it usually has backing vocals (but not in the case of the example below), usually has the same words each time and typically contains the title of the song. This is from the movie Help:
Another contrasting section, called a 'bridge' or 'middle-eight' can appear as well. Here is where strong harmonic contrast can appear. In the next song, the bridge starts with the words "Life is very short...". The verse is the beginning music and ends with the repeated refrain "We can work it out".
In instrumental music you don't have the option of repeating music with different words so other devices become important. Common to both vocal and instrumental music are the basic ideas of repetition and contrast. Some music is repeated and in between different music is used for contrast. The contrast can be melodic, harmonic and rhythmic. Notice the shift into triplets for a waltz feel in the bridge of "We Can Work It Out". In larger instrumental forms harmony becomes more and more a structural device that can be used both for contrast and unity over larger stretches of time. Starting in one key, then modulating to a different key or keys is how you can construct a piece several minutes long. That's something for another post!