From: yaroslavvb@gmail.com   
      
   There are several papers that define it.   
   Here's one: K. Rose, E. Gurewitz, and G. Fox. Statistical mechanics and   
   phase transitions in clustering. Physical Review Letters,   
   65(8):945--848, 1990   
   http://www.citeulike.org/user/yaroslavvb/article/105632   
      
   Basically hard clustering means point x either belongs to cluster y or   
   doesn't. On other hand with soft clustering you are allowed to have x   
   belong to cluster y with probability 0.5 and to cluster z with   
   probability 0.5   
      
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