To prevent any confusion, I am not considering the Bayesian interpretation here (though anyone is welcome to include that in an answer if it helps clarification).
The Wikipedia page on the "classical definition of probability" states that
The classical definition of probability was called into question, [and] The frequentist definition of probability became widely accepted as a result of [this] criticism
I did some reading, but I don't quite understand the difference between the classical interpretation and the frequentist interpretation, since (in general terms) they both deal with frequencies.
So what is the essential difference?