누구예요 is composed of 누구+이(다)+에요 누구 mean "who" 이다 means "be" 에요 is honorific ending suffix 누구세요 is composed of 누구+이(다)+시+에요 누구 mean "who" 이다 means "be" (으)시 is honorific addition 에요 is honorific ending suffix You should add ‘~(으)시’ to verbs/adjectives in which the acting person deserves respect, regardless of who you are speaking to. You can add ‘~시’ to word stems ending in a vowel and ‘~으시’ to stems ending in a consonant. Once ‘~(으)시’ is added, the verb/adjective gets conjugated as usual as if the stem ended in ‘~(으)시.’ Generally, 누구예요? means "Who is he/she/it?" (asking the listener about someone else) 누구세요? means "Who are you?"