OP, both plex and mediabrowser are back ends and function as library tenders for whatever media you have stored. They also both do transcoding, which is what some people refer to when they say "stream." If you transcode you most likely lose your audio format (dolby digital, DTS-MA, etc) and your video will have its quality altered. People transcode in order to get media down to a bitrate that both the end device (such as a cell phone) and the network (such as a WAN connection where you're watching a video from a hotel that's stored on your server) can handle. Most people do not transcode (that I know of) within their household. I'm unsure of how plex handles transcoding vs. simply using something along the lines of samba to read the file, but mediabrowser usually prefers to just "direct" you to the file which is shared over samba, then you can play it directly from where it is stored as though it were on the same computer as it's playing on. This is the same as simply navigating your network to the media on the other PC and playing it from there.
Plex does the same thing. It only transcodes if the target device doesn't support it or networking issues.