Haha, gstreamer is a the libraries that are responsible for media playback.

MPlayer is another media player. It doesn't have a GUI to start usually (but it has a GUI for playback). Not sure if it uses the gstreamer libraries but it handles more media files than the default media player.

libflashplayer is usually for the browser.

Google would tell you more in depth about them. But the description for KMPlayer is accurate. It is a front-end for all of them. It just requires a description of what those are lol.