이다 attaches to the end of a noun to make it the predicate of the sentence. 이다 is used to express that the subject and predicate are the same thing. 이다 can also be used to specify something.

The formal descriptive form is 입니다 and its interrogative form is 입니까?
The corresponding informal forms, 예요/이에요 are the same for both the descriptive and interrogative forms, with the interrogative form rising in intonation as it is pronounced: 예요?/이에요?

When the preceding noun ends in a vowel, 예요 is used，and when the noun ends in a consonant, 이에요 is used.

The negative form of 이다 is 아니다.

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Express that the subject and predicate are the same thing.

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Common conjugation

입니다: Formal, polite form, used in formal writings or respectful speech.
예요/이에요: Polite form, used in everyday polite conversation. 예요 follows a vowel, and 이에요 follows a consonant.
이야: Informal form, used with close friends or in casual settings - See (이)야.

Opposite

The opposite of 이다 is 아니다.

Bridge with verbs

There are a lot of grammar pattern that would normally require to be connected to a verb base (action/descriptive verb) but they can be connected to nouns by using 이다 as a bridge.

Here is an example with (으)며:

졸업식에 가족이며 친구며 아는 사람들이 다 왔어요.

All my acquaintances including my family and friends came to the graduation ceremony.