Back in the day when you wanted to stream some video on the internet you installed RealPlayer, the real plugin was the go to guy for live and normal video streams, it was ubiquitous as todays alternative Flash Player is. Somewhere in the mide 2000′s flash took over as being the default delivery mechinism for streaming media over the internet, potentially helped by sites like Youtube using it.
There were other plugins for your browser, Quicktime was favored by Apple and for a Long time Microsofts choice was MPayer.
All of these technologies have one thing in common, you need to install a plugin within your browser to make use of them, without a plugin you would simply recieve a box telling you something was there, but you can’t play it. This makes the standard web browser a very kludgy thing with all these bolt ons and plugins needing to be installed.