Everything in the earlier writeup is correct. I'm adding some color.
Though the 1883 explosion of the volcano Krakatoa has been enshrined in the public's mind as the most devastating eruption in recorded history, the 1815 erruption at Tambora makes Krakatoa look like a mere firecracker.
The explosion took place on the island of Sumbawa on April 10, but news did not reach Europe until November. The closest thing to a surviving eye-witness account is a letter by a British merchant in the Dutch East Indies (now Indonesia). This letter was published in The Times of London.
The letter described an investigation by a couple of local captains who had sailed to Sumbawa to see what had happened. "They found the sea for many miles around the island so completely covered with trunks of trees, pumice stone and etc as to impede the progress of the two ships... The crops of paddy have been utterly destroyed over a good part of the island. Great numbers have perished and more die daily." For two days after the eruption the island was dark like night.
And that was pretty much all that was known about the Tambora explosion for the next 160 years. However, in the 1970s, research on the glaciers of Greenland led scientists to reconsider what had happened on Sumbawa all those years ago.
A glaciologist named Claus Hammer found a layer of acidic ash in Greenland ice cores that could be dated to the years after 1815. The only large volcano to have exploded around that time was Tambora. Because of the much better documented Krakatoa explosion, people knew that the ash from Indonesian explosions could reach Europe. This, together with a spike in interest in such explosions thanks to the 1983 centenary of the Krakatoa blast, led to many papers being published about Tambora as well.
Prior to the eruption, the volcano was something like 13,000 feet tall (4,000 meters). It now stands only 9,348 feet (2,850 meters). The mountain now boasts a caldera 4 miles (6 kilometres) across and 3640 feet (1.1 kilometers) deep. In photographs from space, it looks almost like a parody of a volcano.
Haraldur Sigurdsson of the University of Rhode Island, described as the foremost authority on the eruption, estimates that Tambora exploded with the force of a billion tons of TNT. This is approximately equal to the 60,000 times the force of the atomic bomb blast at Hiroshima. It ejected seven times as much pumice, rock and ash as Krakatoa, and 150 times as much as Mount Saint Helens ejected in 1980. On the geologists' Volcanic Explosivity Index, which runs from zero to eight, Tambora scored seven.
As described in New Scientist, between a third and a half of all the people killed by volcanic eruptions around the world in the past 500 years perished in the islands around Tambora that summer. Some were killed right away; at least 8000 people were killed by lava flows. Those who lived on nearby islands (and those who fled from Sumbawa to those islands) suffered as well. All vegetation on the islands of Sunbawa, Lombok, and Bali died. The resulting starvation killed an additional 80,000 - 90,000 people.
The global impact was significant. In China, some 1200 miles to the north, frosts destroyed crops and half the trees died. In the two years following the explosion, China suffered exceptionally cold and stormy weather, with terrible harvests.
In Britain, it rained or snowed almost every day in 1816. Prices on the London Grain Exchange hit record highs. Crops failed in India. In New England, it snowed in every month of 1816. The resulting destruction of crops was one of many factors that contributed to the migration of American farmers to the midwest. Some of the greatest works of Gothic fiction, including Mary Shelley's Frankenstein (1818) and Charles Maturin's Melmoth the Wanderer (1820) were written in the "Year Without a Summer".
The English painter William Turner is famous for painting sunsets. Recently it become clear that he was painting skies full of dust from Tambora, and that the glowing images he saw were caused by sunlight scattering off volcanic dust.
There were further eruptions of Tambora in 1880 and 1967.
In the long run, people living on Sumbawa and its neighbors realized one significant benefit from the explosion -- the residual ash from the explosions acts like fertilizer and has greatly improved agriculture there.
New Scientist, http://www.newscientist.com/hottopics/disaster/boom.jsp
The Gothic Literature Page http://members.aol.com/franzpoet/intro.html