The difference between composing, and just writing a song for me is simple. Songwriting usually involves focusing on the particular melodies, and relationships to words. Composing looks at the entire song as a piece.
Songwriting you work on individual melodies, and it's crucial that you focus on those melodies, and the strength in them.
Composing, melodies are simply transitional points that bring you to the next piece. Often composers steal chord structures, and play homage by transposing other people's melodies, and folk melodies. Not because they are unoriginal, but because the song as a whole is what's important.
I've personally done a little of both. I've never written a note because I can't read, or write music at all. Yet, I have composed full electronic pieces that bare many resemblances to composing rather than songwriting:
http://www.archive.org/download/Nero.../11Chap3NC.mp3