The Woman in Black is a story about Arthur Kipps' encounter with, well, a woman in black, so in that sense it's pretty straightforward.
But think about how weirdly bland the title is. Not "The Haunting at Eel Marsh"; not "Jennet's Revenge"; not "The Murderous Woman in Black." Just The Woman in Black. This benign title almost seems to mirror Arthur's own experience with Eel Marsh House and its ghostly inhabitant. At first, when he sees the woman in black at the funeral, he writes her off as just that—a woman who happens to be wearing black. It's rather a matter-of-fact observation...
... until, that is, we see the subtitle: A Ghost Story. And boy does this subtitle make us ask questions. Does including this subtitle take away from the apparent realism of Arthur's experience? Are we supposed to think it's just one more spooky tale to tell around the fire? Is it a self-conscious nod to its Gothic ancestors? Is it a way of poking holes in Arthur's facade of rationality?