A ghost story is a story including a ghost, the possibility of the presence of a ghost or when characters believe in the ghost. Specifically a ghost is an entity, a spirit that have no body. We can see some of these elements in Rebecca.
After getting married Maxim de Winter and his new wife came back in Manderley, a mansion cut off all the exterior world, where the young bride will be haunted by the presence of her predecessor, Rebecca.
She discovers her when she explores her new house, as if the memory of the dead woman tries to separate her and Maxim de Winter, with the help of Mrs Danvers, the chambermaid, who seems to have a privileged relationship with Rebecca. Everytime we feel Rebecca's presence in the movie Mrs Danvers appears. Also we have the feeling the woman is everywhere, as if she was at differents places in the same time. She have a look everytime, everywhere.
In the movie we often see Lady de Winter taking a walk in the house. In these moments shadows pass on her face. Sometimes a window is open when it's not supposed to be or curtains move like if someone animate them. She meets Rebecca's initial everywhere as a remembrance of her.
The return of the sailing boat may signify Rebecca is coming back for separate the newlyweds. Maxim de Winter undersands this really fast because at the end of the movie he says that Rebecca will always win.
The way people talk about her let us believe she's still alive. Also even after her death nothing have moved in the house. The decoration is always the same. Her room is still perfect and as tidy as before. It's like a sacred room where the young bride is afraid to enter. Indeed she only goes in after quite a long time.
Thus we find some features of ghost stories, especially the possibility of the presence of a ghost and the belief in this ghost. It's not said cleary but Lady de Winter is scared by this presence. All of this let's suppose a ghost story.
Sara Fassi & Lou Igier