xine is a stand-alone mediaplayer and more importantly a shared multimeida library (xine-lib) which is used as a back-end for numerous GNU/Linux media players such as Totem, Gxine and KMPlayer. Xine is even used to allow preview of video files in the KDE file dialog boxes.
 Xine - the mediaplayer
Xine a great media player for viewing long movie files. It is a stand-alone player which plays almost all known audio and video formats and it has all the basic features you would expect from a media player.
The main obstacle for using Xine as a all-round media player is the playlist: It is totally crappy. It does have a playlist and you can add files to the playlist using the --enqueue option, but that is about it. Moving files in the playlist is virtually impossible, adding files is a total pain and the "playlist" barely qualifies as a playlist.
So, in bullet summary, if you want to view a movie, then Xine is a great player. But if you want to add 20-30 music videos to a playlist and sort them in a given order.. then Xine is not what you want.
 The library
Xine is excellent for playing a movie file, but the variety of media players based on the library (xine-lib) are better for other purposes (i.e. playlist usage).