IBM server series

P series: POWER series (Performance Enhanced With Enhanced RISC)
To run AIX server

• IBM Power Blade servers
• IBM Power 520 Express
• IBM Power 550 Express
• IBM Power 560 Express
• IBM Power 570
• IBM Power 575
• IBM Power 595

i series: To run ios
Power Systems including AS/400, iSeries, and System i

Z series: Mainframe computers (developed only by IBM)

http://www-03.ibm.com/systems/z/hardware/index.html

DS series: for data storage

X series: x86 architectures INTEL processors;
x86 servers for Windows and Linux
e.g for HMC (Hardware Management Console) server