Hi. They both mean "who" as used in "who are you?". But "dare" is very direct and considered impolite or too direct if used directly to the person you are asking who it is. Examples: If you are at home, and someone knocks on your door or someone calls you on a telephone, it's better to say "donata desuka" (who is it?) since you don't know the person at at all. Let's say that someone stole your bicycle and you found that guy and you confront him. Then you can say "Omae ha dareda?"(Who are you?, very direct and lack of respect). If you ask about a third person to someone else, you should say "dare" as you are not asking the "who" question directly. "Ano-hito dare?" This means "Who is that person" and this is ok. To sum up, it is good to use "donata" in most situation if you ask for the identity of the person directly. Note: grammatically, both are ok. It's just the politeness. The Japanese language has many different words with the different level of politeness like this. I hope this helps.