Bridge: The third melody and the one that is -in most song structures- appearing only once. It is better if the listener gets a "tiny" surprise in the bridge; that can be a change in the chord structure accompanied by a smooth shift to another scale...etc. However, the bridge melody always ends by shifting back to the original mood and repeating the chorus all over again. As for the bridge's lyrics they usually represent a conclusion or a flash back to the whole song, this adds to the surprise. Sometimes, -especially in rock- the bridge is just a solo with no lyrics.