If a songwriter wants to create a song that hopefully, will be recorded on an artist's CD so he can make money from it, he knows he must arrange it in a structure that makes sense to the artist and eventually listeners. He doesn't need to Invent a structure for his new creation, he just needs to choose from several existing structures. He knows whichever form he chooses, his song will probably have an intro, 2 or 3 verses, several choruses, maybe a bridge, and/or an instrumental section and an ending.
The verse is the vehicle for conveying the story that is the song. Its major responsibility is to lyrically and musically, "set up" (or lead to) the chorus. The chorus is the central idea of the song and the verse is the "information" about the main idea. The second verse is more information about the main idea, information that should lead the listener to the conclusion
Each line of each verse should move the story along without repeating the same information. The melody should be essentially the same from verse to verse to create familiarity. A melody that stays relatively the same makes the listener feel comfortable and this comfortable feeling allows them to listen closely to the lyrics and follow the story. A repeating melody is easier for the listener to remember. After they hear it a few times, even if they are not trying to remember it, they probably will.
The chorus contains the "nut" of the song, the central meaning, the essence of the story. Choruses like "I Can Love You Like That," "Old Time Rock and Roll" or "The Wind Beneath My Wings" are memorable choruses. The "hook," the catchiest, most memorable part of the song is most of the time in the chorus. Songs can have other hooks, but the main one is usually the chorus. Verses normally concentrate on details, while the chorus makes a broader statement that bears more repetition. The melody of the chorus repeats each time we hear it. The song's title is usually contained in the first and/or last line, and possibly more. The lyric and melody are usually the same each time.
The bridge relieves the "boredom factor." It is usually placed about 2/3 of the way into the song. (After the second chorus in a verse/chorus form) That's normally when people may begin to tire of the song and need a break of some type. The bridge wakes the listener up and helps them to refocus on the song, and can add new story lines. The bridge can also be a guitar, piano or other instrument solo. This is the chance to introduce a new melody to keep things interesting.
Hooks are also important in a song. A hook is any part of the song that "hooks" the listener causing them to remember parts of the song. Many times the title or chorus is a hook. A hook can also be a clever lyric, a good vocal part, even a sound or effect added by an engineer in the final mix.
The form or structure of a song is critical to it's success. Make sure you understand song form before you begin writing your masterpieces.