Short answers, pulled from the story.
A land mine is an explosive weapon concealed under or on the ground, designed to destroy or disable enemy targets that pass over or near it. It is mostly a casing filled with a main charge, set off by a firing mechanism such as a pressure plate that triggers a detonator and then a booster. Triggers can respond to pressure, movement, sound, magnetism or vibration.
Anti-tank mines are designed to disable tanks and other vehicles, while anti-personnel mines are designed to injure or kill people. Anti-tank mines are larger and normally require about 100 kg of pressure to detonate, which prevents infantry from setting them off. Anti-personnel mines are often built to injure rather than kill, increasing the enemy's evacuation and medical burden.
The Ottawa Treaty is the 1997 Convention on the Prohibition of the Use, Stockpiling, Production and Transfer of Anti-Personnel Mines and on their Destruction. It came into force on the 1st of March 1999 and has been signed by 164 nations, though China, Russia, and the United States are not signatories. It covers anti-personnel mines and requires existing stockpiles to be destroyed within four years of signing.
The United States did not sign the Ottawa Treaty because the treaty lacks an exception for the Korean Demilitarized Zone. Along with China and Russia, the United States may hold tens of millions of stockpiled anti-personnel mines and is not party to the convention.
Land mines can remain dangerous for many decades after a conflict has ended. There is a common myth that mines become inert and harmless after a few years in the ground, but this is false. Self-destruct mechanisms exist in some modern mines but are not absolutely reliable, and most mines laid historically were not equipped with them.
Before gunpowder, buried spikes served many functions of modern mines. The Roman Empire used goads, lilia pits with sharpened logs, and abatis of felled trees with sharpened branches. The caltrop, a four-spiked weapon 12 to 15 cm across, disabled soldiers and mounted forces and was used by the Jin dynasty at the Battle of Zhongdu against Genghis Khan's army.
From 1999 to 2017 the Landmine Monitor recorded over 120,000 casualties from mines, IEDs and explosive remnants of war, with the all-time estimate exceeding half a million. In 2017 at least 2,793 people were killed and 4,431 injured, with civilians making up 87 percent of casualties and children under 18 making up 47 percent.