how to differ from RAM and ROM

Since i buy my first mobile phone Xiaomi in 2011, a question have been surrounding me, what is the RAM and ROM?

when i go to the mobile shop, some sellers introduce me the mobile phone is very good, with 6G RAM and 64G ROM, but when i ask him why RAM and ROM can not be together, then he can not explain any more, who else can explain to me here?

thank you very much

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