Okay, I'm not going to assume anything but I would think that a good percentage of us knows what a song fic is...if not, it's easy enough to figure out. It's quite prolific in fan fiction.
You hear a song on the radio or on that new cd you bought. The song is just awesome and it makes you think of Sirius and his angst in being in Azkaban while Harry's growing up. Specific passages of the lyrics especially trigger this idea. So you scribble down the words (or go out to the internet and find them), sit down in front of your computer and let the images flow.
You use those lyrics to start particular settings in the story to get the right mood going. Well, since you've done that you have to give the band credit in the disclaimer, right?
Well, baby you just wrote a song fic.
Pretty common place actually but in reality, not the best way to do it. I mean, if your audience has no idea who you're band is or has never heard the song, then the beauty of the song is lost. Yeah the lyrics are there, but the tone of the song that inspired the imagery isn't. More often than not you're inspired by one or both the lyrics and the tune.
That doesn't stop song fics from coming out of the woodwork, and on a whole, they are some of the best peices of fics out there. Many writers claim to making "soundtracks" in which they write too. A certain character has to be written to heavy metal, another to instrumentals.
Music is often used not only for characters but also for style of writing. If you are writing a hot action scene with fast paced fighting or chasing, then some of the Bond soundtrack music might be just right. A romantic love scene might be inspired by soft, romantic ballads.
One of the songs I've seen used many times in varying genres of fan fictions is Loreena McKennit's "Dante's Prayer". Actually just about anything by Loreena is fair game, so hauntingly beautiful and lively her peices can be. As a sample of "Dante's Prayer" here are a few lines:
Cast your eyes on the ocean
Cast your soul to the sea
When the dark night seems endless
Please remember me
Then the mountain rose before me
By the deep well of desire
From the fountain of forgiveness
Beyond the ice and fire
The words are a poem in themselves and music is equally beautiful, but anyone not familiar with the music can still get a sense of the haunting beauty of the lyrics. It definitely gets the point across of lonliness (to me anyway). Her website http://www.quinlanroad.com has some of her melodies and all the lyrics to her songs on there. Though you may or may not know the melody and tune, you can read the words and be affected by them. Thus the use of them, in say, Harry sitting in an window sill, thinking about his parents and the family life he never had.
You will never hear me say, don't use songs, because I'm so guilty of using songs in fics and for inspiration of fics, that would be majorly hypocritical. Just be careful as to the nature of the songs you use *in* the story you are writing. Keep in mind your audience as well. What are the odds that your audience, who might be primarily foreign to your country will know the folk song that's common in your country (i.e. How many Malaysians are going to know the American song "Dixie" completely?) You might use it, but not in it's entirety and you'd definitely want to give an explanation about the song and it's origins. A pop song or a well-known song like "White Christmas" can be safely used, no matter the audience, in normal cases, no matter the nationality.
Never NEVER (and as an archivist of several websites I see this constantly and it irritates me) try to pile the whole song, including the "oooooo" and the "yeah yeah baby" lyrics at the front of the story. It's almost a guarantee to make people skip it the song altogether and go straight to the story. If that happens, then the point of having the lyrics there is defeated. Try to break them down throughout the story, use them as scene openers.
For example, using "Dante's Prayer" again:
Cast your eyes on the ocean
Cast your soul to the sea
When the dark night seems endless
Please remember me
Harry sat on the window sill. His mind was focusing on the snow drifts building up on the castle's towers but on a warm October night. The events of his parents' death were well-known to him, having heard it in his mind because of the Dementors but also from friends of his parents, yet somehow he felt that if he concentrated hard enough, he could remember all on his own. The memories of a one year old baby, sleeping peacefully in his crib that horrible night, eluded Harry, and feeling lonely and depressed, he slipped off the sill and into his warm bed.
Then the mountain rose before me
By the deep well of desire
From the fountain of forgiveness
Beyond the ice and fire
Sitting in the Quidditch pitch, a large shaggy black dog shivered in the cold, but the human part of his mind, for he was an animagus, was blocking the misery of the weather around him. Like his godson far above in the castle, Sirius Black was remembering the same awful October night, feeling the cold dread seeping into his bones and making him shiver far more than was necessary in his snowy white environment. He wondered vaguely if James and Lily could see him and Harry, if they understood that Sirius was doing the best he could in keeping Harry safe given the circumstances of him being a wanted criminal. It seemed an unsurmountable task, trying to stay one step ahead of the Ministry of Magic and the Dementors, trying to keep his godson safe, and helping the Hogwarts headmaster, Albus Dumbledore, in the ever-going fight against the evil that took Harry's parents.
Another 'never' is placing the entire song at the end of the story either, especially if you've used bits and peices throughout the story. The important parts were used, there's no use rehashing them. Give the audience the important stuff, and leave the extraneous stuff to yourself. Giving the readers all the lyrics isn't going to help them any and if they are interested there are plenty of places online to find the lyrics themselves if you have accurately given the correct band and song title.
The bonus to using song fics appropriately and skillfully is turning people onto to new musical styles, arrangements and musicians. So think of yourself as not only advertising the characters you are writing about (i.e. Harry Potter and the gang) but also the music that inspires you!