A militia is an informal, part time, or temporary armed force which differs from conventional forces in that its members are generally regarded as civilians mobilised in an emergency. The British Home Guard during the Second World War was a militia force. Militias generally operate only on their home territory. However, formations such as the United States National Guard and the British Territorial Army serve as adjuncts to the regular forces and may be deployed overseas, and are not militias.
Militia in the US
When the Second Amendment was written during the founding of the United States of America, the included the Right to Bare Arms. The Right was written as a precaution that should there be a danger imposed within the borders of the States, there would always be people ready to fight against the threat.
The usage of the word "Militia" is key here. During the early age of America, the word meant: "Any able-bodied male citizens between 18-45 with access to a gun." In Modern America, the age and gender limit has been changed to include women statement remains the same. All Americans have the right to carry weapons as they are all part of a Militia. As shown by the modern 10 US Code §311.