Varies from state to state, in TX, it is an unarmed civil defense force and medical unit. One of the oddities in the US Code is that the only "militia" a state is allowed to have is its federally authorized state defense force.
[h=1]32 U.S. Code § 109 - Maintenance of other troops[/h]
(a) In time of peace, a State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, or the Virgin Islands may maintain no troops other than those of its National Guard and defense forces authorized by subsection (c).
(b) Nothing in this title limits the right of a State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, or the Virgin Islands to use its National Guard or its defense forces authorized by subsection (c) within its borders in time of peace, or prevents it from organizing and maintaining police or constabulary.
(c) In addition to its National Guard, if any, a State, the Commonwealth of Puerto Rico, the District of Columbia, Guam, or the Virgin Islands may, as provided by its laws, organize and maintain defense forces. A defense force established under this section may be used within the jurisdiction concerned, as its chief executive (or commanding general in the case of the District of Columbia) considers necessary, but it may not be called, ordered, or drafted into the armed forces.
(d) A member of a defense force established under subsection (c) is not, because of that membership, exempt from service in the armed forces, nor is he entitled to pay, allowances, subsistence, transportation, or medical care or treatment, from funds of the United States.
(e) A person may not become a member of a defense force established under subsection (c) if he is a member of a reserve component of the armed forces.
