[citation][nom]phatboe[/nom]What exactly are these Z-Series mainframes used for and what OS is it running?[/citation]
IBM zOS, IBM RHEL (Red hat linux) and IBM's Virtual OS Host thing.
You can run java applications, COBOL (legacy mainframe stuff), databases, websites, EVERYTHING.
These mainframes are a thing that puny-brained desktop users can never fathom. One can have redundant processors set aside on each processor board or the processors can be assigned to specific tasks, like differing types of I/O channels and such.