"Do you know her?"
你知道她(吗) is not correct. 知道 is used for facts, 认识 for persons, places, movies, even things you can be "acquainted" with personally. Many languages distinguish these (for instance French: savoir/connaître; Spanish: saber/conocer; German: wissen/kennen).
你认识她。 = You know her (personally, you met her).
你知道她不喝茶。= You know (the fact) that she does not drink tea.
你认识北京。= You know Beijing (personally, you have been there).
你知道北京很大。You know (the fact) that Beijing is big.
你认识《哈利·波特与阿兹卡班囚徒》。= You know "Harry Potter and the Prisoner of Azkaban" (personally, you have read the book or watched the movie).
你知道《哈利·波特与阿兹卡班囚徒》是一本书。= You know (the fact) that "Harry Potter and the Prisoner of Azkaban" is a book.
吗 (Ma) = What?
Above is the literal translation of the character 吗 . It is usually appended at the end of a statement to convert it into interrogative.
他们是学生 (Tāmen shì xuéshēng) = They are students.
他们是学生吗? (Tāmen shì xuéshēng ma?) = Are they students?
她是医生 (Tā shì yīshēng) = She is doctor.
她是医生吗? (Tā shì yīshēng ma?)= Is she doctor?