Why is the Yogācāra school called 'mind only'? I've heard the Yogācāra school of Buddhism called 'mind only'. What does that refer to? Does this school believe that the mind has a real inherent existence but nothing else does? Alternatively is it a more metaphorical designation or perhaps a provisional description of a deeper notion or maybe a name given to it by other schools (in the same way as Hinayana was used by Mahayanan schools)?
To fill out R Barzell's comment more and to venture my own answer in the hope of being corrected. It seems like the most satisfying explanation of this is from this source 


  the reality we think we perceive does not exist except as as a process
  of knowing. Phenomena, anything that can be experienced, have no
  reality in themselves. At the same time, there is no "experiencer" who
  experiences except as a process of mind.


My further understanding of this though is that the Yogācāra school was a very practically based school and any philosophical discussions would have been secondary to meditation and personal enquiry so this kind of statement would have been provisional on this. This would contrast with the rival Madhyamika school which had a greater emphasis on philosophical discourse. Peter Harvey in an Introduction to Buddhism (pp129 2nd Edition) says of this


  Madhyamika has an analytical, dialectical approach [...] emphasizing
  wisdom; the Yogacara's emphasized samadhi (meditative concentration)