“您贵姓？”, which means “May I know your name?”. This is a respectful and polite way of asking the name of a person. The answer is not “我贵姓…”, but “我姓…”
“你叫什么名字？”, which means “What’s your name?”. It is used by elders when they want to know the names of young people or between young people. One shouldn’t use it, therefore, when one wants to know an elder’s name or when one needs to show respect and politeness to the other party.
“他（她）叫什么名字？”, which means “What’s his(her) name?”. It is used for asking another person’s name. One shouldn’t say “他（她）贵姓？”
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Please post in the comment if you have any words that you do not understand. Happy learning !