Outside of the game, they were just named for ethereal beings.
Canon-wise, they generally had a reason for being named whatever by the marines. i.e.
Wraith - spirit that you die after seeing
halo meaning - tank that blows the sh!t out of you after seeing. (you die)
Banshee - Whining spirit that screams and kills people
halo meaning - named for the faint(er) whining noise that banshees make as the fly, and how they'll kill you.
Whenever a thread was hijacked and there were big quote boxes and lots of flame, I was there!
GT: I DFang I