When Assyria conquered the Jews of the Northern Kingdom of Israel in 721 B.C., they deported a large number of Jews to other parts of their vast empire, and brought into the area many people from other nations they had conquered.
Since many of these people intermarried with Jews who remained in Israel, the population became of mixed nationality, no longer purely Jewish. These people also followed religious practices that were not wholly Jewish.
Because the capital city of the Kingdom of Israel was Samaria, the people came to be known as Samaritans.
When the Jews of the Southern Kingdom of Judah returned from exile in Babylon in 538 B.C., the Samaritans actively opposed their desire to rebuild the city of Jerusalem. So, by the time of Jesus, there was lingering hostility between the Jews and Samaritans.
Since the Samaritans lived between the Jewish territories of Galilee and Judea, many Jews going to and coming from Jerusalem had to pass through their land. Samaritans were often hostile to these travelers.
From The Little Black Book: Six-minute meditations on the Sunday Gospels of Lent (Cycle A).
