To understand dLeyna, you first need to understand a little bit about DLNA. A detailed overview is provided here, but in essence, the DLNA is an industry body that publishes guidelines that define how consumer devices can share media content. The DLNA defines two main types of media devices. Firstly, there is the Digital Media Server, or DMS for short, which allows a user to consolidate all their media content on single device, e.g., a network attached storage device, and then to make this content available on the network so that it can be browsed from any of the user's other devices, e.g., his PC. Secondly, there is the Digital Media Renderer, or DMR for short, which allows a remote device such as a mobile phone, to use the output capabilities of the device upon which the DMR runs. Examples of DMRs are smart televisions, loudspeakers or games consoles.
dLeyna is an opensource project that provides high level APIs for creating DLNA enabled applications. The dLeyna APIs are much higher level and easier to use than the underlying UPnP APIs upon which dLeyna depends and they require little or no knowledge of the UPnP/DLNA standards. By DLNA enabled applications, we mean applications that allow users to take advantages of the services offered by DMSs and DMRs. For example, a user could use a DLNA enabled application running on his tablet computer to instruct his television to begin streaming a video stored on his network attached storage device. Alternatively, he could make a photo stored on his mobile phone appear on his television so that it can be seen by everyone in the room.
So why would anyone want to build DLNA applications? Simply put, DLNA devices are becoming ubiquitous. All windows PCs support DLNA out of the box as do most smart tvs, set top boxes and games consoles. By 2016 it is estimated that there will be 3 billion DLNA certified devices. We need DLNA enabled applications to consume the services offered by all these DLNA devices and we need dLeyna to build these applications.
The other interesting thing to note is that these services can be consumed in an unlimited number of ways. The DLNA guidelines describe two classic DLNA applications, a Digitial Media Player (DMP) which can locally play the content it discovers on DMSs and a Digital Media Controller (DMC) which can instruct a renderer to play content stored on DMSs. But we are not bound by these use cases. In general any application that allows users to download or to create media content can become a DLNA application and hence can benefit from the dLeyna APIs.