A MediaServer is in UPnP speak a device that provides media content and (directory) information about it.

In addition it can provide an interface to upload and modify/remove content and a subsystem that records and stores broadcasted content.

Coherence has several MediaServer backends:

file-system
This is a simple one, which just exposes the files on the file-system as content in the given directory structure. Its primary use is acting as a test-bench for the MediaServer functionality, but regardless of that it is operational.
There is support for album art display, just taking the first .jpg or .png it finds inside a media directory as the cover image.
It has experimental support to transcode ogg audio files to something DLNA MediaRenderers can understand.
Elisa
That backend connects to the Elisa Media Center and exposes the content of its media database.
Flickr
That backend connects to the Flickr service and exposes for now 100 of the most interested in pictures of the last day.
[gallery2]
connects to a Gallery2 photo repository (Work in Progress)
Apple HD Trailers
Exporting Apple HD Movie Trailers to UPnP clients on your network.
iRadio
This backend connects to Shoutcast and transforms the internet radio streams there into "normal" UPnP audio objects. (young and buggy, ymmv)
Ampache
A MediaServer connecting via the XmlApi to Ampache and exposing the content of the media database there.
Rhythmbox
A MediaServer embedded in Rhythmbox, exposing its media database.
Meta Tracker
A MediaServer connecting via DBus to Meta Tracker and serving the indexed files there.
[last.fm]
Listen to your last.fm radio with an UPnP client (work in progress)
Axis-Cam
This is just a simple demonstration on how to act as a proxy for a non-UPnP service, announcing the video stream of an AXIS IP cam.
MediaDB
A more advanced MediaServer as the file-system storage, using a sqlite db as its backend, with ID3 tag support and cover art retrieval. Experimental yet, with additional dependencies described in the source.