Samaritans = descendants of the people left in northern Israel after the Assyrian invasion of Israel in 721 B.C. Intermarriage of this group with the Assyrian people resulted in a group of part-Jew/part-Gentile people.
They were despised by the Jewish people for a number reasons:
They were not "fully Jewish"
They built their own temple at Mt Gerizim
They had their own unique copies and interpretations of Torah
They had their own high priest and sacrificial systems
They believed they were the true descendants of Israel