The structure of a song consists on the way the composer organizes the ideas or melodies in different parts or sections of a song. Let`s see what are the main sections in a classical pop-rock song.
                                                          THE STRUCTURE OF A POPULAR SONG:
When you build a song you have to think about the parts that this song
has to have. 
a)   
Introduction: It`s an instrumental part just
before the lyrics starts. 
b)   
Verse: It`s the part of the song that
tells the story. It moves the story forward. This section always repeats the music but the words change. 
c)    
Pre-Chorus: It's a short transition section. It prepares the coming climax which is the chorus.  It has the same music and the same words. 
d)   
Chorus: it`s the most catchy part of the
song. A good chorus has to stick in your mind. It`s repeated several times. Music and words are the same and the title of the song is usually
included in this part. 
e)   
Instrumental break: It is an instrumental section or
interlude during a song. Its main function is to take a break from the main
parts of the song. 
f)    
Bridge: It normally differs melodically,
lyrically and rhythmically from the other parts of the song. It gives the song
contrast before the final chorus.
g)   
Coda: It’s an Italian word for tail.
It`s an optional addiction to a song and it represents the end of a song. 
         Click on this link to listen to a song as an example of a classical song structure.