Martial law is a law declared by the military rather than a police force, fire department, or other civilian government. The types of scenarios when martial law may be used is in response to a rare emergency or crisis. When it is declared, martial law means the actions which would normally be taken by emergency services or local authorities are handed over to the military.
In regions or countries that are controlled by elected persons or governments, martial law means that those representatives are no longer in power.
This only occurs in the most extreme cases so that order can be restored quickly and so that one organization is in control and can assess the situation effectively. There is the potential that when martial law is enforced, that power may not be given back.
When martial law is being enforced, people living their daily lives in that particular city or country are also directly affected. Rights, such as free movement, free speech and privacy, can be suspended in order for the military to restore order.
Also, structures or processes such as the justice system may change from one that is controlled by criminal and civil law to a military tribunal system, which means people could be arrested and charged for violating new laws in place, such as curfews.
Again, martial law is extremely rare. Cases, when it could be implemented, are during times of civil unrest, protests, or coups. It was used in 1892 in Idaho when a group of mine workers blew up a mill, and the National Guard was sent, under direction from the governor, to restore order.
When countries are experiencing disasters, such as hurricanes, snow events and tsunamis, what is more commonly used is a state of emergency, which carries a similar level of seriousness, but the government and local authorities retain power.