Well, they are a mix of Jewish, patermally ancestrally Jewish, and Kuthim who came in after the Babylonian exile (possibly/probably mixed descendants of "lost" tribes). If you recall, the children of non-Jewish women (who refused to convert) were exiled from Jerusalem, and had to go somewhere, so they mixed in with this pseudo-Jewish group that had come in during the exile.
From this mishmash, came Samaritans. Obviously, being expelled, they couldn't have the Temple in Jerusalem, so they changed it.
FWIW, they are considered "Jewish" in Israel under the law.
I've actually seen sacrifices at Mt. Gerizim. They've been studied closely on the assumption that one day the gold-domed pagan abomination sitting on the Temple Mount will one day be removed and the real Temple could be built. (The assumption being that the Samaritan practices would be similar, and so might be able to fill in holes in the text and traditions.)
You can see the sacrfices on Youtube, in face.