What exactly is a Samaritan?
According to Tanach (the Bible), the Samaritans were a mix of different people brought over from Mesopotamia to Shomron or Samaria in Israel by the Assyrian Empire, after the Assyrians exiled the Kingdom of Israel as punishment for rebelling against them.
An Assyrian inscription also attests to certain Arabic tribes being exiled to Samaria.
The Tanach tells us that they settled there and eventually assumed a form of the Israelite religion, but when the Jews returned from the Babylonian Exile to Judea, they rejected the Samaritans (there are different ways to understand the reasoning brought in the Book of Ezra). This rejection led to harsh clashes between the two groups for many years. Jewish sources, including Josephus, as well as archeological findings, tell us that eventually the Samaritans built a temple on Mt. Gerizzim in Samaria, to contest the Jewish Temple in Jerusalem. This was eventually destroyed by the Hasmonean prince John Hyrcanus. To this day the Samaritans deny having had a temple in the Hellenistic era.
On the Samaritan side of things, they hold that they are descendants of the Israelites and were the only ones that remained faithful to the one true holy site in Israel, Mt. Gerizzim, where the tabernacle was placed when the Israelites entered Israel, while the rest of the Israelites strayed and erected tabernacles and temples elsewhere (Shiloh, Jerusalem, etc). Eventually the Assyrians exiled them, and so before leaving Samaria, they hid the tabernacle away, to this very day. Shortly after they were exiled, they were able to come back, unlike the rest of the People of Israel, who had to remain in exile. And so they've been living in Samaria ever since, though at some point they also began building communities in other parts of Israel. Today they have two communities: One on the outskirts of Shechem and the other in Holon, and until recently, were dying out.