UPnP is a standard allowing all kind of devices to communicate with each other in side your LAN. As far as I know, it is not possible to do UPnP over the internet except building a VPN connection allowing you to do a remote login.
Today you won't hear much about UPnP, it has become a subset of DLNA.
As far as I know gapless playback is not part of the standard.
Pretty irritating if you play live albums or classical music.
Look for vendor specific solutions if you want gapless playback.
UPnP (Universal Plug and Play) is a bit of a pun on Plug and Play.
This time it is about devices recognizing each other over the network.
They exchange their capabilities.
A couple of years ago it was most of all Plug and Pray.
Today most DLNA implementations are mature.
A lot of smart TVs and smart phones support this protocol.
If possible try to find out if the device is DLNA certified.
If you enable media sharing in WMP, you can play the content on all devices in your network supporting the UPnP protocol.

The library pane in WMP will show all available devices.
In my case another PC running Vista and my NAS, a Qnap 109 running TwonkyVision

Both PCs run Vista.
It take some time before the library is complete, probably 20 minutes but then there is a one to one correspondence between de content on both machines.
For samplers I use Various Artist as the Album Artist and the Contributing Artists for the name of the performer.
I saw a triple entry of a sampler.

Corrected it on the server, using refresh on the client and after a moment the corrected content is displayed.

My NAS comes standard with the TwonkyVision Media server, in this case running version 4.4.6
In the communication between WMP and TwonkyVision obvious the contributing artist is uses as the artist. All the samplers are split. You get as much albums as there are tracks with different artist.

You can configure the music tree of TwonkyVision but Vista ignores this.
I have 3378 contributing artist in my collection. Using Vista – TwonkyVision only 775 are displayed in WMP. Likewise for songs and albums, can't get the entire collection to be displayed in WMP.
Asset is built by Illustrate, the maker of dBpoweramp.
There is a free and a premium version.

The browse tree is configurable

Transcoding (on the fly conversion) audio formats allows you to play formats
not supported by the client.
More details can be found in the manual.
Asset in WMP11
A paid UPnP server.
This is version 4 as running on my NAS.
It also has a browser interface: http://IPNas:9000/
You can browse your collection in various ways.

If needed you can configure this.
This configuration allows me to browse by composer.
If I select a composer, I get the compositions (album) and per album the movements.

Unfortunately all the options to configure the tree are no longer available in server version 5.
TwonkyMedia is a media player for Windows
TwonkyServer Version 6 - $19.95/€14.95
TwonkyServer is available for Windows PCs, Windows Home Server, Mac and Linux
MinimServer is written in Java and requires Java 6 to be installed.
MinimServer is available as downloads for Windows x86, Windows x64, Mac OS X, Linux x86 and Linux ARM. There's also a QPKG version for simple installation on QNAP NAS devices.
MinimServer is designed with classical music in mind. The simple and powerful browser interface indexes your music by composer, composition, genre, conductor, orchestra and soloists so that you can explore all aspects of your music collection and quickly find what you're looking for.
Price: freeware.