UPnP (Universal Plug and Play – Streaming audio)

The name is a bit of a pun on Microsoft's Plug&Play.

Plug&Play means you plug a device into a computer and it works without any interference from the user.

UPnP does the same but this time over the network.

UPnP devices auto-discover each other and exchange there capabilities.

Although not limited to streaming AV, in practice streaming audio and video is where UPnP is about.

Today UPnP has become part of OCF.

Of course UPnP has its own nomenclature:

Server: streams the music to UPnP clients in the network,
Renderer: a device with can play the music received from the server
Control Point: the remote control.

If you are looking for a UPnP server:

• check user forums before you buy. A lot of these server software is very buggy and/or cracks under load.
• check if it is DLNA/UPnP certified.
  

UPnP is incorporated in DLNA.

Today you can also access your media at home over the internet using UPnP.
Unfortunately it is a security risk as you have to open ports in your router.

Windows

XP is UPnP compliant.

XP can be used as a server only,

Vista can be both a server and a client (renderer/control point).

If you enable media sharing in Vista (an option in Windows Media Player) the PC becomes visible as a media player in the network.

Win7 and higher is fully UPnP/DLNA compliant.

OSX

OSX doesn't support UPnP, Apple of course has its own proprietary protocol; Airplay

You need  third party UPnP software on OSX

Android

There are UPnP/DLNA compliant media players for Android.
This allows you to use your smart phone as a remote control for your media player.
A couple of examples can be found here.