The name for this type of device isn't clear, but the device functions is similar to a Set Top Box for cable TV. Here are the characteristics:
1. It "feeds" a TV or receiver with audio, video, and pictures. This connection can be made with an HDMI cable (preferred), composite, component, optical, etc. This connection is just like any other A/V device (DVD player, for example).
2. It connects to the network. This can be wired, wireless, etc.
3. It supports DLNA (but some UPnP devices will also work).
3a. It must be more than a DLNA Server. It must also be a Renderer, meaning it can "play", and not just "serve" files. Some "media servers" support DLNA, but do not support the renderer feature. They are useful but don't belong in this category.