Short answers, pulled from the story.
Viking success came from fast, shallow-draft warships, unnerving battle tactics, skilled hand-to-hand fighting and a cultural expectation of fearlessness. Small bands could strike a coastal target and disappear before defenders or reinforcements arrived.
The spear was the most common Viking weapon, inexpensive and effective, with a wooden shaft between two and three metres long. Vikings also carried axes, bows and double-edged swords around 90 centimetres long, though swords were reserved for the elite because of their cost.
Viking longships had a shallow draft of about a metre, letting them beach directly on open sand instead of using fortified harbours. Their light, flexible construction and low mast let them travel far up rivers such as the Seine, the Loire, the Rhine and the Thames.
Rollo was a Viking leader who seized what is now Normandy in 879, a claim formalized in 911 when Charles the Simple of West Francia granted him the Lower Seine. His settlement became a forerunner to wider Viking expansion into farmland and trading posts across Frankish, English and Russian territory.
The Great Heathen Army was a Viking force, formed from bands already active in Britain and Ireland, that swept Anglo-Saxon rulers from power in 865. By the 870s it had brought most Anglo-Saxon kingdoms, including Wessex, under Viking control, shifting Viking activity from raiding toward conquest and settlement.
Berserkers appear in Viking sagas as warriors granted magical, injury-proof powers by the god Odin, though those claims are exaggerated. The term is rooted in a real trance-like state in which such fighters would engage in reckless fighting, and Christians in Frankish and English territory feared them as satanic.