Vikings
The Scandinavian raiders, traders, and settlers who for three centuries reached from Newfoundland to Baghdad and founded states from Normandy to Kyiv.
Viking is a job description, not an ethnicity. In Old Norse it meant something close to going raiding, and most Scandinavians in the period never did it. The convention is to date the age from the sack of the monastery at Lindisfarne in 793 to the failed Norwegian invasion of England in 1066, which is tidy and roughly right.
What made it possible was a ship. The Scandinavian longship had a shallow draft, a keel, a single square sail, and oars, which meant it could cross open ocean, run up onto a beach without a harbor, and row far up rivers that no other seagoing vessel could enter. That combination collapsed the distinction between a naval and an inland threat. A monastery two hundred miles up a river was no longer safe.
Raiding, trading, and the difference
The raiding is the famous part and was genuinely brutal, but it was one activity among several and often the same men did all of them. The Scandinavians were also the most effective long-distance traders in Europe, moving furs, walrus ivory, amber, and above all enslaved people, taken from the British Isles, the Slavic lands, and each other, into the markets of the Islamic world.
The scale of that last trade is easy to underrate. Enormous quantities of Islamic silver dirhams have been found in Scandinavian hoards, which is the archaeological trace of an economy where the primary export was human beings. The word slave and the word Slav share a root for related reasons.
Where they went
West, they settled Iceland from the 870s, reached Greenland around 985, and around the year 1000 established a short-lived camp at L'Anse aux Meadows in Newfoundland, making them the first Europeans in the Americas by roughly five centuries. The Greenland colony survived for four hundred years and then died out in the cooling climate of the fifteenth century.
South and west, they took a large part of England, the Danelaw, and were bought off in what is now France with the territory that became Normandy in 911, whose duke's descendant conquered England in 1066. Sicily and southern Italy fell to Norman adventurers in the same century. The Viking age ends with the Normans, who were Vikings with a French accent.
East, the Rus went down the river systems of what is now Russia and Ukraine, established themselves at Novgorod and Kyiv, and founded Kievan Rus. They reached Constantinople repeatedly, sometimes to attack it and eventually to serve in it: the Varangian Guard was the Byzantine emperor's personal corps of Scandinavian axemen for centuries.
Why it stopped
Not because they were defeated. The age ended because the conditions changed. Kingdoms in England, France, and Germany consolidated enough to defend their coasts and rivers. Scandinavia itself consolidated into three Christian kingdoms whose kings preferred to tax trade rather than license raiding, and Christianization removed both the religious difference and the raiding culture. The Vikings did not disappear. They became Danes, Norwegians, Swedes, Normans, and Russians.
The echo
Two things they left are still visible.
The first is a map. Normandy, and through it the Norman conquest of England, the Russian and Ukrainian founding story out of Kievan Rus, Iceland as a polity, and Dublin as a city all trace to Scandinavian ventures. The founding argument about Kievan Rus, whether it was principally a Norse or a Slavic creation, is still politically live in the dispute between Moscow and Kyiv over who inherits it.
The second is a warning about sources. Almost everything written about the Vikings at the time was written by literate churchmen whose institutions were the primary targets, which is roughly like reconstructing a war from one side's casualty reports. The horned helmet is a nineteenth-century opera costume. The archaeology has been steadily replacing the caricature with something more interesting: a maritime commercial civilization that raided when raiding paid and traded when it did not.