The first should be 'details' because 'are' should be paired with a plural noun. e.g. What are the details of this... If you are using 'what's' or 'what is', then the singular form should be used. e.g. What is the detail... or What's the detail... The word 'is' is used with nouns in the singular form. The word 'are' is used with nouns in the plural form.

When people are speaking they don't always use perfect grammar.