A song is words+music, and for a song to be successful it has to have great lyrics and great music.
Great lyrics are simply a mix of easy, nice words revolving around an interesting topic. The good thing is that lyrics don't have to go by any rules; it is simply a matter of practice...May be there are some rhyming "rules" but those can be hardly called rules. We will discuss rhyming later.
As for great music, you don't have to create a complicated masterpiece..! Actually, great music in this context means simple, catchy melody and well-structured chord patterns. We will also see about music later.
OK. Now we got music and lyrics but that means nothing if a catchy melody and a number of nice words were just scattered all over your song...Music and Lyrics need to be put into a concrete Song Structure...
A song usually has three short melodies, two of which are repeated several times…
Chorus: The chorus is a lyrics-music combination that is repeated with little or no change throughout the song. Usually, the chorus's lyrics include the song title and give the listener a general idea about the song's topic. The chorus music includes the catchiest melody in the song and it is usually the part that your listener will remember from your song.
Verse: The verse has the second melody of the song; this melody hardly changes throughout the song. The verse's lyrics are actually what usually change; as the verses usually give details about the song's topic. Notice that no matter how the lyrics change they all have to fit the same melody, without having to look "artificial"...This actually is a great challenge for the lyricist ;).
Bridge: The third melody and the one that -in most song structures- appears only once. It is better if the listener gets a small surprise in the bridge; may be a change in the chord structure or a smooth shift to another scale...etc. However, the bridge's melody almost 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 an instrumental solo with no lyrics.
But how are those three parts arranged in the song? Well, you got Chorus, verse and bridge...Yes, arrange them in any way you want, but don't get too messy or your listener will get lost. To make it easier, songwriters have come up with these agreed-on song structures. Most of the songs follow three structures…
verse / chorus / verse / chorus / verse...etc.
verse / chorus / verse / chorus / bridge / (verse) / chorus
verse / verse / bridge / verse
Now, about the verse / verse / bridge / verse ... This structure takes a tricky melody for the verse; it has to be catchy and longer than the usual verses. Moreover, you have to spice it up every time you repeat it with a new thing (like a new instrument in the background, or a back vocals line...etc.). I personally like this structure because I feel it gives a certain freedom with the lyrics, the other two structures don't give. Besides, this structure gives room for musical intros and outros, not to mention the opportunity to add some musical breaks (between the verses) without having to worry about making the song too long.