The same as in english. Young woman and girl. Young man x boy It is objective when you start using one over the other.
yes. Dívka and holka both mean "girl". Slečna is usually a young lady and thus such a person can be in both categories, dívka and slečna, depending on the speaker feelings. It is close to Miss in English. It also means she is not married. Miss Marple in Czech is Slečna Marplová.
There is also a word "paní" (and "pán" for males) that is between žena and dáma. It is close to Mrs. and Mr. but does not require the name to follow it. It usually gets translated as "lady" for lack of any closer expression. Dáma is somebody who has a class, is nicely dressed...