Like any operating system, it can be thought of like an onion, with many layers. In the outer layers are found bundled applications, such as the Macintosh Finder. There may be several available APIs. Apple's Toolbox, Carbon and Yellow Box would be examples. It is the portion that deals directly with the hardware.
Drivers plug in to the kernel to allow it to adapt to deal with new types of hardware.
Furthermore, each of these layers may be broken down into other layers, depending upon their implementation. For example, in Mac OS X, underneat the developer APIs are a UNIX personality and the Mach kernel. The functions of the Mach kernel, device drivers, and UNIX personality are the least interesting parts of the OS from the perspective of a typical Macintosh user. Their interests are in the user experience, the graphical user interface and bundled applications provide this. We are not asking Apple to open these technologies.