According to Apple
The current version of Mac OS is Mac OS X (pronounced "Mac O-S ten"). ... . Major releases of Mac OS X include versions 10.0, 10.3, and 10.4. There are also updates (sometimes called "dot" releases) for each major release, such as versions 10.2.8 and 10.4.2.
This does present a problem because it isn't correct to write Mac OS X.6. All references to Mac OS X 10.6, are easier to pronounce as "Oh Ess [Ex] ten dot/point six," rather than saying, "ten," twice.
As for Darwin,
The Darwin layer of Mac OS X comprises the kernel, drivers, and BSD portions of the system [...]. Mac OS X extends this low-level environment with several core infrastructure technologies that make it easier for you to develop software.
Apple purchased NeXT for their XNU kernel, which is a hybrid kernel forked from CMU's Mach microkernel. BSD is an implementation of Unix originally released through UC Berkley. Aqua is the rendering engine for the user interface. The Apple Finder is a system application that always runs for user access to files and the desktop. OS X is the sum of these and other parts.