Pronounced [ka-cow], cacao is how raw foodists pronounce the same exact thing you probably call cocoa. Except, if you see or hear cacao, it probably signifies a raw chocolate powder, sauce, bar, butter, etc. And that difference is huge!
Chocolate carries a lot of nutritional qualities (in small infrequent quantities) and has strong antioxidant qualities. Nutritionist and raw food advocate, David Wolfe, had a high-quality cocoa powder compared to that of raw cacao powder in a labratory and they found raw cacao to contain 367% more antioxidants. Look for raw cacao (you can still say cocoa if you choose) powder at your local health foods store.
So why the difference in pronunciation and spelling? Apparently most of the world refers to cocoa as cacao. As legend has it, an English trader misheard and then mispelled the cacao bean and today (in the English speaking world) we have cocoa.
Most chocolate consumed in the United States is derived from cocoa beans dried by large high-heat ovens. Raw cocoa beans are dried with heat of 100 degrees or less or sun-dried to retain it's full nutritional value.
So now you know why some say cacao and some say cocoa and why some seek out raw vs. processed.