The basic point is that both have same meaning and the original form is -이다. But there are two dirrerences. First, with -이야, you can make a question(raising intonation) and a declarative sentence(falling intonatipn), but 이다 makes just a declarative sentence. Second, -이야 is informal land spoken language, so we can speak this verb to friends. But -이다 is written language so we can see it in the book, usually.

"-이야" and "-이다" are usually used to make nouns into predicates like "ialah" in Malay. "-이야" is a spoken form of "-이다", which can be used among close friends. Dia ialah seorang guru. ☞ 그녀는/그는 선생님이다.(just statement, neutral) ☞ 그녀는/그는 선생님이야.(in spoken, informal and intimate) \^o^/