The most recent revision of the POSIX standard is IEEE Std 1003.1-2000 (cross-referenced to the appropriate version of ISO document 9945-1). The standard is distributed as four volumes describing a particular component of the system. The Base Definitions section lays the groundwork for all the specific terms and concepts used in the rest of the spec. System Interfaces is half of the real meat of this definition, everything dealing with how the system itself is supposed to behave. The other half is covered in Shell and Utilities which describes all the standard programs that are expected to be on a *NIX system. All the design decisions and such can be found in the final volume, Rationale (Informative).
While there are four volumes in the POSIX standard library, most POSIX conformity references are to specific subsections (based off the 1996 revision of the specification). These are: