The word Vikings comes from Old Norse (the word víkingr) and they were a group of people that lived primarily on the sea. Coming from the Nordic areas of Denmark, Norway, and Sweden, they existed from the late 8th century to the 11th century. This period is commonly referred to as the Viking Age. It is an important part militarily and expansion wise for Scandinavia, Estonia, the British Isles, France, and even Sicily. They were a strong, fierce people devout in their beliefs and terrific raiders.
The Vikings were at home on the sea, with exceptional seafaring skills as well as their staple long ship. At times their influence spread to the Middle East, Africa, and even Central Asia. They spread far and wide, going into European Russia to being the first people besides the Native Americans to discover North America.
Evidence of Vikings being around the world is found very often, such as evidence of Vikings in Baghdad. They would often trade with people on the Volga, which is the primary river in Russia, going from Russia into the Caspian Sea. They would trade a wide variety of things, such as furs, seal fat, and even slaves. They would also frequently visit Byzantium, working as mercenaries for hire for the Byzantine Empire as an imperial body guard and were referred to as Varangian.
Vikings are often shown to be monsters with horns, and complete savages. These are common misconceptions that were brewed in the 18th century, close to 700 years after their existence. The 20th century has not aided in this, with common representations showing them still with horns, which they never had, and being complete brutes, when they had beautiful architecture, art, and culture.
The earliest known Viking raids is documented in the 790s and extends to 1066. They would use the Norwegian and Baltic seas to travel south, and were given specific territories to lord over France in the 10th century. Two Vikings even went on to become King of England, Sweyn Forkbeard from 1013-1014 and then his son, Cnut the Great from 1016-1035.
No one is certain why the Vikings wanted to expand so rapidly and so wide, but there are some theories. Charlemagne, who was King of the Franks, wanted to covert everyone to Christianity by force, so there could have been resentment towards the Europeans as they resisted against the conversion from their 'pagan' ways. Some just say that the Vikings were opportunists, striking at a good moment when there was chaos and profiting for themselves. There was very little naval activity from any country besides the Vikings, and most countries, like England, were broken into several parts.
The Viking Age ended in the way it was destined to fall, European influence. In the late 11th century, the Catholic Church was making headway and gaining more power and influence. Towns appeared all over Scandinavia, and three kingdoms, Denmark, Norway, and Sweden, all had taken form. Their economies started taking a monetary form, and markets were forming, following the English and German models. Their wealth coming from the Middle East and England had come to a halt. Raids would continue into the 12th century, however most of the Scandinavian rulers would have new, Catholic goals like taking back Jerusalem for the Catholic Church.
The Viking Age was a very interesting age with no one sure as to the reason for it. They shaped so much because their influence was so wide though they stayed primarily on the sea and the coast. While they didn't wear horns, and weren't barbaric savages, they were brutally tactical and a diverse people.