z/OS is a 64-bit operating system that runs on IBM's traditional mainframe hardware, also known as System z.
z/OS, along with zLinux, zTPF, z/VSE and z/VM, is an operating system which runs on IBM's System z hardware.
This tag should be used when asking questions related to the specific operating system z/OS, associated tags may be mainframe, cics or db2 (provided you're talking about the mainframe versions of those products).
A descendant of OS/360, a primary design premise is backward compatibility. Application programs dating from the 1970s continue to run unaltered on modern z/OS systems.
IBM provides the z/OS basic skills education Knowledge Center, click on the table of contents link for a list of topics.
