I must agree with ap123 on this one - Scrivener is a brilliant program that's built with writers in mind. I was thinking about using MS Word before I got started on my first book, but then I asked myself, "There has to be software aimed at making the management of all the data that I would organize around my creation a lot easier." Because let's face it: Depending on the genre of your work you may likely have:
- various web sources that you're using for research,
- texts copied from wikipedia or quora for future reference (more potential research, in other words),
- photos and images for inspiration,
- character sheets for your main and supporting characters.
And if you are one for managing an outline for your novel, you'd have that, too.
I wanted a solution that handled ALL of that, and I got that with Scrivener. It usually costs $40 (one-time purchase; you can sometimes find it on sale for as much as 50% off), but it's worth it.
EDIT: Regarding cloud storage, I've set up my instance of Scrivener to sync all content to my Google Drive account, allowing me to access and work on my book from both my desktop (at home, where I usually get most of my work done) and my laptop (because sometimes grabbing a table at the nearest empty lounge with wifi serves as a great place to work).