Music server

A server is a computer somewhere in a network. There is a program (the server process) listening for clients. If the client ask for something, the server will start to do what the client  asked for (that’s serving).


A music server is a computer where the music is stored and has a process running which enables clients to browse its content and to stream selected songs to the client over the network.


Any computer with the appropriate software can do the job but often a NAS is preferred simply because it is a small computer so low on power consumption (you don't need much performance to stream audio).
The music server software is often UPnP compliant.

 

A typical example is the Squeezebox.

It accesses the internet over the home-network for internet radio and your own collection is stored on a computer running the Logitech Media Server-software.
In general this technology is called streaming.

Dedicated audio PC

Often a PC is used for audio only.
It is stripped from all what is not needed for playback.
Although it is used for direct playback so no server functionality here, often it is called a music server. Sending the audio over e.g. the USB to a USB DAC is often described as streaming.

In my opinion it is what is, a dedicated audio PC.

Music server

When a product is advertised as a music server in general it is a computer with a soundcard, a hard disk and software having the functionality needed to act as a media player.

However, it has the look of traditional audio gear.

A computer in disguise.