I have recommended Scrivener — for me the best writing software ever — multiple times. It is hard to give a correct impression of the advantages, but the posting Scrivener — A perfect program for dissertation writing tries to give an overview. It is my most frequently read posting. However, some people might like a video and there is an excellent one linked on the Scrivener Trial page, or available on YouTube. The creator of Scrivener, Keith Blount, gives a 10 minute introduction of the program:
Highly recommended, especially if you combine it with a good outliner like Circus Ponies Notebook, and available for Mac and PC (Windows).