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)?