A militia is a group of people not in the military who do many things that the military does. This includes collecting weapons, training soldiers, defending people or land, policing an area. Militias are often made up of people who used to be in the military, but have left for some reason.
Some militias are organized by a government, like the United States National Guard. This is a group of state militias first created to help with natural disasters and other emergencies. Others are not connected to a government, and may even be illegal. One common definition of a militia includes all men who are able to serve as soldiers.