They dump your clothes into a sheer laundry bag and wash it in hot water along with everyone else's, and the same for drying.
I remember one cruise I bought one of the sale T-shirts and had it washed (I cannot stand my clothes to smell like stale smoke). When it came back the graphic was already starting to peel off. Cheap shirt, life goes on.
I love using the service to wash my cruise clothes midway through the cruise, or near the end so I have less to wash when I get home.