Jews are descended from the tribe of Judah which is the tribe that King David and Jesus descended from.
The Jewish Virtual Library's article Who Is a Jew? says:
In common speech, the word "Jew" is used to refer to all of the physical and spiritual descendants of Jacob/Israel, as well as to the patriarchs Abraham and Isaac and their wives, and the word "Judaism" is used to refer to their beliefs.With regard to your belief that some tribes are excluded from being known as Jews, the site continues:
Technically, this usage is inaccurate, just as it is technically inaccurate to use the word "Indian" to refer to the original inhabitants of the Americas. However, this technically inaccurate usage is common both within the Jewish community and outside of it...It's worth going to the link and reading the full explanation there. Jewish identity/ethnicity is derived from Abraham, Isaac and Jacob, who predated the division of the land into two kingdoms.