These people are members of an underground militia which holds suspicion that The Government is going to declare martial law, seize everybody's guns, and perhaps cede national sovereignty to the United Nations to form a One World Order, or implant everyone with microchips to make it easier to track them, and start sending patriots like them to prison camps any day now — but not on their watch! Particularly unsympathetic examples will have those displaying neo-Nazi sympathies. The methods employed by the more fanatical of them may even include brazen violence and terrorism toward the government.
While the militia movement has antecedents going back decades (many militias themselves claim the "Minutemen" of The American Revolution as spiritual predecessors), most of these characters appeared during The Nineties in American media, particularly after the Ruby Ridge incident, the Waco Siege and the Oklahoma City bombing, which involved government confrontations with supposed Real Life versions of these characters. The truth about them is a bit more complicated; see the Analysis page for more.
Militias died down after the surge in patriotism that accompanied 9/11 (although some of the more radical groups contended that the whole thing was an inside job), as well as Clinton's replacement with a supposedly right-wing president, but the last couple of years have seen militia membership surge to levels not seen since The Nineties. Most of this has been attributed to a bad economy, increased "security" such as the Patriot Act over the past decade, the election of a Democratic President and anger over health care reform and immigration.
They are seen in many different ways by different Americans, considered evil by some, but right by others. Disagreements even rage within their own number: a great many follow libertarian or traditionalist ideals, opposing the unnecessary violence and edgy attitude of the Neo-Nazis. The majority of the militias do not actually engage in terrorist activity like the media often portrays them of doing, but rather merely seek to train for trouble and keep an eye on the ever-expanding Federal government.