It's a bit complex subject involving two separate concepts of politeness. I will call them "polite" and "honorific". 이에요 = a polite sentence ending form of 이다 ("is"). It shows courtesy to the other person (the listener). 이세요 = a polite and *honorific* ending form of 이다. It shows respect toward the subject of 이세요, which may or may not be the listener. Here is how the two phrases are formed. 이에요 = 이다 + -어요 (politeness suffix) -> 이어요 -> 이에요. (이어요 becomes 이에요 for easier pronunciation) 이세요 = 이시다 + -어요 -> 이시어요 -> 이셔요 -> 이세요. (again, 이셔요 becomes 이세요 for a similar reason) 이시다 = honorific version of 이다. You add the -시 suffix to the stem to get an honorific version (이다->이시다, 가다->가시다) Here are usage examples of the two. 1. 저는 학생이에요 = "I am a student" said in a polite register - the subject is "I", so it uses the non-honorific "이다". 2. 저의 아버지는 공무원이세요 = "My father is a public official" - honorific 이시다 is used because the subject is "father". Politeness is usually expressed through the sentence ending form, while honorific forms can appear anywhere in the sentence. - 공무원이신 우리 아버지는 아침에 일찍 출근하세요 - uses honorific form "이신" (from 이시다), in a polite sentence. - 롱무원이신 우리 아버지는 아침에 일찍 출근하셔 - uses an honorific from in a plain sentence (talking to a friend). - 중학생인 제 동생은 아침에 일찍 학교에 가요 - similar sentence, but the subject is a younger brother, so a plain from 인 (from 이다) is used.

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.’ e.g. 그는 학생이에요. 저 분은 선생님이세요. (Express honorific for the subject) 누구예요? means "Who is he/she/it?" (asking the listener about someone else) 누구세요? means "Who are you?"

If you want to talk gentley or politely, you can use 이세요 instead of 이에요.