Gothic, Mystery
Creepy old manors, supernatural events, terrible secrets coming back to haunt people (sometimes literally), and all things scary and weird: yup, The Woman in White is your textbook gothic novel. The title alone pretty much screams gothic.
Horace Walpole's The Castle of Otranto is considered the original Gothic, and authors like Mary Shelley, Edgar Allan Poe, and even Jane Austen (see Northanger Abbey) hopped on board the genre's bandwagon. Collins definitely continues the gothic tradition with this novel, but he does it with a twist. Instead of supernatural events, we get real-world dangers. Collins takes the threats and creepy atmosphere typical of gothic novels and transplants them to more realistic settings… which, if anything, makes it creeper. It could happen to you.
The Woman in White is actually considered one of the very first "sensation" novels. Sensation novels focused on shocking subject matter (adultery, murder, kidnapping, etc.), but placed it in normal, even domestic settings for contrast and shock value. Check out our "Best of the Web" section for more info on sensation novels.
Mystery
The Woman in White is distinguished by a lot of "firsts." It's Collins's first hugely successful novel; it's considered one of the first sensation novels; and it's also considered by some to be one of the first detective novels. Way to intimidate a bunch of aspiring writers, Collins. Jeez.
Walter pretty much acts like a private investigator in Part 3, and the way he runs around gathering evidence and stories is something that later becomes a hallmark of the detective fiction genre. At any rate, The Woman in White definitely has a ton of mystery; we get major secret after major secret—from Anne Catherick's parentage to Sir Percival's big secret, there are more mysteries than you can shake a stick at in this big ol' novel.