'Which' is used to indicate that there is a limited range of choices to choose from. 'Which car is this (the Mercedes or the Rolls) (mine or yours or the gardener's)?'
'What' indicates an unlimited range of choices. 'What car is this (a Toyota, a Nissan, a Ford, a Jag, a BMW, a Chrysler, a Honda,...)?'
Having said this, Jack, I will now tell you that speakers make individual choices on which of these question words to use, so the two are in practice interchangeable.
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