PCs can take alot of different forms, servers are just one example of a high powered PC used in a corporate or networking enviroment.  A PC usually has all of it's components built in.  Srvers usually rely on external appliances for things like storage or disk back up or tape, networking with other computers, and networking with other storage devices.  the storage devies themselves are appliances that usually have other computers built into them.  The disk drive we have in our own PC has it's own green board with a CPU and RAM built into it.  An appliance that a server would used would in a way be more of the same.   

Storage appliance

raw platter storage 18 TB (18,000 GB) 

CPU too large, fast multiple-core server CPU

Memory 16 GB fast server RAM

peak thoughput 200 MB per sec. x4

sustain throughput 221 MB per sec. typical

Usable capasity 2 Pb (2000 TB, or 2 milion GB)