NOTE: by the time the priesthood of the Kohanim (כוהנים) was established, the term B’nei Yisrael (בני ישראל) — Israelites — had already supplanted the term Ivrim (עבריים) — Hebrews — to describe the Jewish ancestors.
A rabbi is a Jewish clergy person whose status is derived from a great deal of study. There are different types of rabbis, including pulpit rabbis who lead a shul (congregation) for prayers and other events; there are also rabbis who teach, study, or engage in other professions. The title is earned, not inherited.
A kohen is a Jewish person whose father was a kohen, with paternal lineage being kohanim all the way back to the first priest, Aharon, the brother of Moses. This position is hereditary, not earned. Kohanim are religiously obligated to remain ritually pure (a requirement dating to the Temple service), and this entails certain stringencies that do not apply to other Jews who are not kohanim. Kohanim are typically honored by being called first to the Torah and are able to perform certain functions in shul (such as giving the priestly blessing) that are distinctive. However, in the present, the distinctions between a Kohen and someone who is a Levi or Yisrael are not so pronounced.
Historically, the Kohanim were responsible for running the Temple service and occupied a much more central function in the religion than they do now.
When it comes to seeking advice or religious guidance, one goes to a Rabbi.
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