I am writing a song, but I don't know how to name it. What is, for example, the difference between:
Prelude (comes before something, but Chopin wrote opuses of only preludes)
Ballade
Nocturne (night music?)
Etude/Etude-Tableaux (study/study-something)
Impromptu (written off the top of the head?)
Romance
Fantasy
Moment-Musicauex (spelling?)
or anything else I am forgetting.
Help from the seasoned composers?
Like the others said, you can call it anything.
Here, however, are some simple parts that define the form of a piece:
Prelude: Can come before a piece, but sometimes written in whole opi because they are meant to go in front of something else at the discretion of the performer.
Ballade: Suggests some sort of story. Program music, if you will.
Nocturne: Yes, it is night music. Meant to be conveyed in a dreamy aura, and in free form (although Chopin's tend to follow an A-B-A type of form)
Etude: Study of a particular pianistic difficulty.
Impromptu: Not written off the top of the head, buit written so that it
sounds like you wrote it off the top of your head. An impromptu should always sound like it's going somewhere, and you don't know where.
Romance: Think of love and poetry- maybe something you would serenade your love with (although it would be pretty hard to move a piano all the way to beneath your significant other's bedroom window.

)
Fantasy: Completely free-form. Think of it as a super-Impromptu.
Moments-Musicaux: These are just little ditties. Bagatelles and ecossaises are the same way. They just expand a small idea slightly.
The bigger forms are a lot harder to write for unless you happen, like me, to think in large structures. Most of what I write is in some version of sonata form. I just can't leave the little pieces at what they are: little pieces.
Phil