Dina D'Malchusa Dina

The gemara in several places in shas (Gittin 10b, Baba Kama 113a, Baba Basra 54b, Nedarim 28a) has a statement of Shmuel that "Dina D'Malchusa Dina" - the law of the land is law. Generally, when the gemara refers to this halacha it is a reference to the fact that a government has the ability to tax it's citizens according to halacha. The Poskim do, however, expand this halacha to other areas besides taxation. Based on several sources, the halacha may be expanded to such areas as minting currency, keeping law and order, and even possibly to other areas like the validity of government shtaros. For a nice discussion of the many sources on this issue see Rabbi Hershel Schachter's article on Dina D'Malchusa Dina printed in the Journal of Halacha and Contemporary Society.

In terms of the lomdus behind Dina D'Malchusa Dina, there are various opinions as to what precisely is the source. The gemara in Sanhedrin 20b has another statement of Shmuel that "anything said in the Parshas HaMelech (in Sefer Shmuel), a melech is mutar in that". Basically, Shmuel warns the people of all the powers a king will have over them. According to this opinion in the gemara that parsha in navi is to be taken l'halacha, and a melech as actually permitted to do those things. Tosafos there in Sanhedrin says that this rule is only by a Jewish king over the land of Israel. However, others (see Meiri in Nedarim and the Gra in Choshen Mishpat 369) say that this applies to all governments. If so, the Parshas HaMelech would, in fact, be the source for dina d'malchusa dina.

The Dvar Avraham (1:1) in an essay on Dina D'Malchusa Dina proposes that Dina D'malchusa Dina really stems from the halacha of hefker beis din hefker. The sources for that din are explicit in the gemara in Gittin 36b.

The Ramban (brought in the Magid Mishneh Gezailah 5:13) says that dina d'malchusa dina only applies to old taxes, but not new ones the king imposes. R' Schachter explains that he must hold that dina d'malchusa dina is a hischayvus midaas, the people have knowingly been mechayev themselves in the tax. By new taxes there is no such hischayvus. The Rashbam in Baba Basra 54b also seems along these lines, that the source of the halacha is from the minhagei medinah.

The Ran in Nedarim writes that dina d'malchusa dina stems from the fact that the king can expel his people from his land. It's basically a tax of living in the country. Thus, the Ran writes his famous shitah, that since all Jews are entitled to live in Israel, there is no dina d'malchusa there.

Finally, there is the opinion of the Bais Shmuel that dina d'malchusa dina is only a din midrabbanan.