～はだれですか？ / Who is ~?

だれ

This means "Who". As in English, this word is usually used in a question.

Aは　だれ

This is "AはB / A is B" grammar. This means "Who is A.". This sentence sounds a little strange when the intonation of the ending part doesn't go high pitch to make it sounds a question, because "だれ" should be used in a question.

Aは　だれ？

I put the question "?" mark at the end. In Japanese, we don't have a question mark, but I use it so that we know this sentence is a question easily. So, this sentence is a question with the ending intonation high pitch. This sentence means "Who is A?".

Aは　だれ　です　か？

As usual, we put "です" to end the sentence in polite way, and put "か" to make this sentence a question obviously. This sentence means "Who is A?" as well.

あのひとは　だれですか？

"あのひと/that person". So, this is asking "Who is that person?".

あのかたは　どなたですか？

This sentence means the same as "あのひとは　だれですか？". But, this is using more polite way. "あのかた/that person". this "かた" is correspoinding to "ひと". "かた" is more polite way to say "person". At the same time, "どなた/who" is used instead of "だれ/who". "どなた" is also more polite way for the word "who".