What the heck is DLNA and why you should care about it? I'll keep this simple.
If you have a lot of media stored on your PC--namely music, movies, and photos--DLNA makes it possible to stream that stuff to other devices in your house.
What kinds of other devices? TVs, Blu-ray players, media boxes, and even smartphones and tablets. What's cool is that you might already own the gear you need. In that case, all you'd need is some free software for your PC.
That software effectively turns your PC into a server, making your media libraries available to compatible devices that connect to it. Among the popular media servers for Windows: Plex, TVersity, and Twonky.
On the device side, you may find that some manufacturers incorporate a branded version of DLNA. For example, many LG Blu-ray players and TVs offer a feature called SmartShare, but that's really just DLNA under a different name. Same goes for Philips' Simple Share and Samsung's AllShare.
If you have one of those devices, the manufacturer might steer you to a particular media-server program--like, say, Samsung's AllShare for Windows. But you may still be able to use a different server app if you like, including any of the aforementioned. As long as the device on the receiving end is DLNA-compliant, it should work.
One area where DLNA can get tricky, especially when you're trying to stream video, is codecs. That refers to the way a video file is encoded, which can vary quite a bit depending on the source. This is especially true if you're downloading files from the likes of Bittorrent. If the video uses an unusual encoding format, it might not stream properly.
Alas, the only way to find out is through trial and error.
If you're eager to get your feet wet in the wonderful world of DLNA, I recommend installing Plex for Windows and then the Plex app for your Android, iOS, or Windows Phone device. Plex also works with Roku boxes and LG and Samsung TVs (those with "smart" features, that is). Give it a try, then let me know what you think!
Contributing Editor Rick Broida writes about business and consumer technology. Ask for help with your PC hassles at email@example.com, or try the treasure trove of helpful folks in the PC World Community Forums. Sign up to have the Hassle-Free PC newsletter e-mailed to you each week.