just what ilaughead said... finishing touches, the way it's sewn... it's probably a bit more complicated, or it takes more time, experience... whatever...
the difference is that instead of having the clothes sewn together in a sweatshop in china, the hire professional seamtresses that work on the pieces individually.
the result is that the garment's stitching is of a better quality (because the person has spent more time on and put a lot of attention to it) and a higher price (because you're paying a person per hour).
i do hope for the love of clothes they remembered to use god quality fabric. couture technic means nothing if the fabric's cheap.