Song form is an agreed convention only it is not cast in stone. You can vary form if you like but it is a good idea to know what the conventions are, so when you break them you have a good and valid reason for doing so. Because if you don't you will end up with a lessor song.
Song form is to some point genre dependent. This is because the function the lyric is designed to perform is different for different genres. The lyrics to a dance song are there only to convey an emotion they tell no story have no beginning middle and end. They are secondary to the music. So what we are talking about here is song form where the lyric plays an equal or more important part in the success of a song.
AB: This is not often used these days and I am unaware of it being used in a hit song since "Hey Jude" and that was a long time ago. It is a more common form in musical theatre. An example is "Somewhere over the rainbow".
The form consists of two contrasting lyrical sections they normally run to twelve bars each. There is no chorus.
So in the Hey Jude example you have the "Hey jude don't make it bad section and the "Anytime you feel the pain" section. Both different rhythmical structures. In the case of Hey Jude the song ends with a three minute long extro but it is still AB form.
AA: Form This also has no chorus but quite often can have a refrain that is a single repetitive line that ends every verse. This is most suitable to story telling. Bob Dylan used this form quite a lot. An excellent example of the form used for story telling is "God help me I was only nineteen" because he does not use the repetitive refrain line after every verse. In the beginning it appears every other verse. So he has stepped slightly outside the form. He has contrasting verse sections, in that verse one and two have a slightly different rhythm and melody. This is different to the AB form mentioned first because it is not a totally unrelated melody, its a variation of the first verse melody. This is quite often a device used when writing AA songs because it adds contrast, making up for the lack of a chorus. The second verse melody variation usually goes up in pitch from the first verse melody to add to the contrast. At the back of the song he reverts to one consistent form for the last two verses which both have the refrain. And he has a good reason for doing this so he can hammer the hook line "God help me I was only nineteen" towards the end of the song. So he doesn't stick slavishly to the form but where he doesn't the reasons for doing so are valid which makes it a great lyric. Here is a link to the lyric if you are not familiar with it. http://www.lyricsmode.com/lyrics/r/redgum/i_was_only_nineteen.html
ABA: Form Verse chorus verse. This is the most common. And it can come in many settings. Like AABABB verse verse chorus verse chorus chorus. But essentially they are all the same thing verse chorus.
ABCB: Form verse chorus bridge. This is essentially the same as the ABA form and can have all those variations, however it is usual to have the bridge section between two choruses.
Embellishments to form. I consider Intros and Extros to be embellishments to form rather than a form unto themselves. Just a note about the term extro, it is convention that the language of music is italian so we talk about legato fortissimo we don't say smooth or loud. For some reason all US song writers and songwriting professors have decided to refer to the extro (which has ex as its latin root, as in exit, exodus, etc.) as "outro" so when I am talking about extros it refers to what you may know as an outro, makes no difference same thing.
It is a interesting technique to use form variation to create intros. Two different choices I am aware of are adding lines to the first verse to set up the song story " I loved her first" you can see an example here where the first four lines of verse one don't appear in the following verses. http://www.azlyrics.com/lyrics/heartland/ilovedherfirst.html
The second choice is to open with a variation of your chorus. A good example is the John Lennon composition "Help"
The chorus is:
"Help me if you can I'm feeling down
And I do appreciate you being 'round
Help me get my feet back on the ground
Wont you please, please help me."
The song's intro is a variation of this.
I need somebody
Not just any body
Wont you please, please help me"
link to help lyric http://www.azlyrics.com/lyrics/beatles/help.html
What is good about this technique is you get a chance to hammer your hook idea. In this intro the hook "help" appears four times.
He uses exactly the same technique in the extro where he repeats "help me help me"
So that is the basics of song form. The main points are to know what they are, so if you step outside the accepted norms you know what you are doing and have a good reason for what you are doing. Also consider your variations of your main idea to use in intros and extros.