POSIX specifies the user and software interfaces to the OS in some 15 different documents. The standard user command line and scripting interface is the Korn shell. Other user level programs/services include awk, echo, ed, and numerous (hundreds) others. Required program level services include basic I/O (file, terminal, and network) services.
Since the IEEE has been charging very high rates for POSIX documentation and not allowing on-line publication of the standards, there has been a tendency toward the "Single Unix" standard, which is open, accepts imput from anyone, and is freely available on the internet.