The Little Ice Age was a period of cooling lasting from the mid-14th through mid-19th centuries. This cooling, which has been confirmed by derived temperature readings from tree rings and ice cores as well as from historic data, brought an end to an unusually warm era known as the Medieval Warm Period, during which wine grapes were grown in England.
It was initially assumed that the LIA was a global phenomenon. It is now less clear that this is true ; for example the reconstruction of temperature in the northern hemisphere over the last 1000 years  does not show a pronounced period of cooling. See Medieval Warm Period for more on this. The IPCC describe the LIA as a modest cooling of the Northern Hemisphere during this period of less than 1°C.
The Little Ice Age brought bitterly cold winters to many parts of the world, but is most thoroughly documented in Europe and North America. In the mid-17th century, glaciers in the Swiss Alps advanced, gradually engulfing farms and crushing entire villages. The Thames river and the canals and rivers of the Netherlands often froze over during the winter, and people skated and even held fairs on the ice. In the winter of 1780, New York Harbor froze, allowing people to walk from Manhattan to Staten Island. Sea ice surrounding Iceland extended for miles in every direction, closing that island nation's harbors to shipping.
The severe winters affected human life in ways large and small. Famines became more frequent (the one in 1315 killing 1.5 million people alone), and deaths from disease increased. The population of Iceland fell by half. The Little Ice Age can be seen in the art of the time; for example, snow dominates many village-scapes by the Flemish painter Pieter Brueghel the Younger, who lived from 1564 to 1638.
What caused the Little Ice Age? Scientists have identified two likely suspects: decreased sunspot activity and increased volcanic activity.
During the period 1645-1715, right in the middle of the Little Ice Age, solar activity as seen in sunspots was extremely low, with some years having no sunspots at all. This period of low sunspot activity is known as the Maunder Minimum. What the precise link between low sunspot activity and cooling temperatures is has not been established, but scientists say that the coincidence of the Maunder Minimum with the deepest trough of the Little Ice Age is highly suggestive of such a connection.
Throughout the Little Ice Age the world also experienced heightened volcanic activity. When a volcano erupts, its ash reaches high into the atmosphere and can spread to cover the whole earth. This ash cloud blocks out some of the incoming solar radiation, leading to world-wide cooling that can last up to two years after an eruption. Also emitted by eruptions is sulfur in the form of SO2 gas. When this gas reaches the stratosphere it turns into sulfuric acid particles, which reflect the sun's rays, further reducing the amount of radiation reaching the earth's surface. The 1815 eruption of Tambora in Indonesia blanketed the atmosphere with ash; the following year, 1816, came to be known as the Year Without A Summer, when frost and snow were reported in June and July in both New England and northern Europe.
Beginning around 1850, the world's climate began warming again and the Little Ice Age may be said to have come to an end at that time.