Juno was Jupiter's wife and sister for the Romans, the way Hera was Zeus's wife for the Greeks. Because the Romans and the Greeks were both descended from Central Asian Indo-Europeans, Hera and Juno probably started out as the same goddess. But the Romans thought of Juno a little differently from Hera. Juno was mainly a goddess of marriage, like Hera, but she also protected towns and villages. Many towns in Italy had temples to Juno in order to get her protection. Sometimes Juno was a war goddess, ready to fight to protect the towns.
The Romans often thought of Juno as part of a group of three gods, Jupiter, Juno, and Minerva. They seem to have gotten this idea from the Etruscans. Many Etruscan and Roman temples were dedicated to all three of these gods together.