Servers in the sense of (1) computers marketed as servers should not be
confused with servers, in the sense of (2) any computing running the server
part of a client‐server system. I see that Wikipedia conflates server (1)
with server (2), that is it: they discuss “servers” as if (1) and (2) was
the same thing.
Servers (1) are not very different from other desktop or laptop computers.
There are some features sometimes found in “servers” that are not found
in desktop and laptop computers, for example: ECC memory, support for more
than 1 CPU, hardware RAID controller in motherboard or as a expansion card,
rack mountable or blade form factor, support for SAS disks.
Almost any computer can be a server (2), you do not need a server (1). Unless
you require a feature only found in servers (1), just ask your friends to
give you any computer they are going to discard. This is good because it
reuses what would otherwise be wasted, it means less money to
megacorporations, less environmental contamination, and may monetarily cost
you from nothing to less than purchasing a second hard server (1) from an
stranger.