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.