Part of the song

Intro

This is the introduction, always appearing at the start of the song. In popular music this typically lasts between 4 - 8 bars, this works best for radio airplay since most songs usually only last around 3 - 4 mins, short and simple, that's the way they like it. Other genres of music, such as rock and dance, generally don't apply this rule. The main job of the intro is to grab the listeners attention. It should pull your listeners in and make them want to hear the rest of the song. It is usually an instrumental form of the verse or chorus.

Verse

The purpose of the verse is to tell the story of the song. Every song has a story or a message to convey and the verse is where you do it. Musically the melody of the verse is the same each time. Lyrically it differs, with each verse developing the story. Dynamically, verses are usually a little quieter than the chorus. Remember, the amount of verses you use depends on what song form you are using.

Pre-Chorus

The pre-chorus happens between the verse and the chorus. It is used to add an additional level of dynamics. Not all songs use a pre-chorus, it tends to be used on an "only if needed" basis. Its main function is to build tension up to the chorus and then the chorus provides the relief. It can really help make your chorus stand out. A pre-chorus is usually kept fairly short, between 2 - 8 bars.

Chorus

This is without a doubt the MOST important part of a song. It is where you convey the main message of the song. It should be the same each time, both musically and lyrically, and the title of the song should also be placed within it. A great chorus is catchy and memorable. After hearing your song only even once, the listener should walk around all day with it stuck in their head.

Bridge

The bridge comes after the second chorus of your song and its main purpose is to add contrast. This could be either a new chord progression that breaks up the repetitive verse/chorus structure, an instrumental solo, new lyrical message etc. Typically not lasting longer than 8 bars.

Outro

The end of a song can sometimes be referred to as an outro. This is when the song fades out gradually or has some kind of big finale. This could be the chorus repeated a number of times with the singer using ad lib.