The tree from which chocolate is derived is the cacao tree, otherwise known by its species name Theobroma cacao. After the cacao seeds (including the beans, which grow inside the seeds) are dried and fermented, they are known as cocoa. From this point, the beans can be processed into cocoa powder, cocoa butter, or chocolate.
So the tree is cacao, and cocoa is the substance made from the cacao tree. Elements of the cacao tree that have not been processed in the manner of cocoa remain cacao.
Don’t confuse either of these terms with coca, the evergreen shrubs cocaine is made from.
As different strains of cacao beans exhibit different flavor qualities, master chocolatiers almost always employ a blend of beans. [Forbes]
It’s a sweet, mildly spicy drink made with couverture chocolate flakes and good-quality cocoa powder. [Sydney Morning Herald]
Raw cacao powder has nearly four times the antioxidant content of regular dark chocolate. [Daily Mail]
Her scent of cocoa butter drifted out onto the water, and the boys could not take their eyes off her. [NPR]