DLNA is an acronym for Digital Living Network Alliance which is a consortium of company's providing products that have the ability to talk to each other via the network.
This is a protocol originally designed by Sony to allow remote streaming of audio, video, and pictures across a local network.
 Technical details
DLNA has three modes of operation. They are: a destination device, a source device, and a control device. Some devices can perform more than one role. For example a Windows Media player can be the source device while a Blu-Ray player was acting both as a controller and a destination. In Windows Media version 12 the Player can also act as a control device and so you could use it to select and send music to a dedicated active speaker system that can act as a destination (Sony does make such a device). A DLNA capable remote control could be used to handle (control) a full network of DLNA devices.
Windows Media Player has preference controls that permit it to act as a DLNA source or turn off that feature. These can be changed if you need to.
The Ace Player app for iOS can also act as a destination and control device. It can also act as a control device that can bridge between DLNA and Apple's Apple TV protocol for a wireless network. The same company also has an app for a Mac.
 Blu-Ray Player
Sony Blu-Ray player and some others can talk to a computer and can be used to view pictures and listen to music (or audio book) that is stored on the computer. Sony calls this feature Bravia Internet. It is also supposed to support video but is fussy about the formats.
Both the computer and the Blu-Ray player are on a single local network but that is not enough to allow them to share data like you would between two computers on the network. What you need is a program running on your computer that can act as a mediator for the Blu-Ray player. It turns out that Windows Media Player can act in this role. It is a new feature in the version 11 that permits it to talk to other DLNA devices. I have this running on a Windows XP system primarily and have tested it on a Windows Vista system. I understand it may work even better on Windows 7 running version 12. It happened magically on my XP system but I had do a bit of work to get it on my Vista. It wasn't hard.
I had my Vista machine up and running and then I turned on the Blu-Ray player and connected it to the Internet. Nothing seemed to happen. As it turns out these two machines are at opposite ends of my house on different floors. I went over to my computer that runs Vista and saw that I had a connection request from an unknown source requesting access to my Windows Media Player. Once I clicked OK then it hooked immediately. I could go back to the Sony and I saw it listed. Now I had two computers on the same Blu-Ray player
What you can do is browse your "My Documents" folder in two sub-folders, "My Music", and "My Pictures". Put all your stuff in these two places for access. It is OK to have more folders inside the two main ones. Once there you can access whatever you want and display or listen to it on your TV that is hooked to the Blu-Ray player. It is simple to use once it is set up. The player has its own ability to categorize entries and you can set up play lists on your computer using Windows Media Player.
Windows seven also has multimedia support for DLNA. You need to share the media app to get this to work. Windows 10 supports this but you will need to use settings to get to the control and turn it on. Bring up the Windows menu and select Settings. Then up in the top right of Settings is a search box; type stream. You will find it will present "Media streaming options" as one choice. This will allow you to turn the feature on or off which will allow DLNA.
 iOS Ace Player
Ace Player for iPad has a feature that has the ability to support DLNA. I was able to view movies, listen to music and even display pictures from the standard Windows 7 locations. I could also use my Apple TV from this app to send these same movies and music for display on my TV. Unfortunately Ace Player does not support this forwarding feature for pictures.