Technically : Curtains are vertical sheets of cloth, Drapes are horizontally run pieces of cloth.
Practicly the terms are interchangeable.
Imagine a single piece of cloth that starts on the floor runs up to a drapery post or valance bx above a windowdrapes across the top of the box making a pretty, then proceeds across another drapery post and down to the floor, That would be a drape. Curtains just hang, and stop at teh top and bottom. I believe there is something I read once that said drapes just hang and curatins are movable...... Hmm I guess I better go read up.