The PA-RISC is a RISC processor developed by Hewlett-Packard. It is currently produced only by HP. At the moment only one OS runs on PA-RISC based machines: HP-UX. There is an effort underway to port Linux to them, though.
The Hewlett-Packard precision architecture reduced instruction set computer. A RISC instruction set is easy to decode in hardware and for which a compiler can generate highly optimized code.