Scientifically, a raven has 17 primary wing feathers, the big ones at the end of the wing.
They are called pinion feathers.
A crow has 16.
So, the difference between a crow and a raven is only a matter of a pinion.
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Actually in biology there is no defined distinction between Raven or Crow (Though many have been proposed like the one you said) mostly this just the relic of British English terms where they were at many places used interchangeably.
In general sense Ravens are used to refer to bigger species of Corvus genus while crow for the smaller ones (Atleast this is how British used it)
