The National Guard and Air National Guard are reserve components of the US Army and US Air Force.
The National Guard structure is what replaced the Militia structure, being created as a federally-funded force under the Militia Act of 1903.
Though federally funded, they are normally under the command of the individual states with Governors acting as commander-in-chief until called to federal duty by order of the President.
They are considered soldiers and airmen at all times and subject to recall to active duty at any time. They are paid based upon their total hours of service per month (prorated based upon US military pay rates). They hold the same ranks as their active-duty counterparts. They are subject to the same regulations as active duty personnel and expected to maintain the same high standards.
Traditionally, they serve one weekend per month and a two-week training period or deployment annually as a minimum requirement, although there are full time Guardsmen and Guard units.
Their primary function is to provide a ready force for a variety of missions ranging from search and rescue, disaster relief, civil actions as may be deemed necessary by their governors and frontline combat units.
They are the largest component of the US Armed Forces and are oftentimes counted in with active forces as "gained units."