USC TITLE 10 > Subtitle A > PART I > CHAPTER 13 > § 311 Defines the Militia as:
(a) The militia of the United States consists of all able-bodied males at least 17 years of age and, except as provided in section 313 of title 32, under 45 years of age who are, or who have made a declaration of intention to become, citizens of the United States and of female citizens of the United States who are members of the National Guard.(b) The classes of the militia are—
(1) the organized militia, which consists of the National Guard and the Naval Militia; and
(2) the unorganized militia, which consists of the members of the militia who are not members of the National Guard or the Naval Militia.
In other words, the Militia consists of all able Citizens. The purpose of the Militia can be found in the oath that all members take on enlistment in the Military. We swear to "Protect and Defend" from all enemies, both "Foreign and Domestic".
Foreign enemies are easy to identify. They wear a uniform and ride in military equipment under the flag of another Country. Domestic enemies are Terrorists and criminals.
You see, in this Country, ALL CITIZENS share in the duty to provide in the common defence of society. By consent, we have deligated this duty to a professional Police Force. But we must remember that because we deligated this responsibility, it does not absolve us from the obligation we have to fulfill it as required.