Vikings were simply sea-borne Scandinavians. Some performed raids in the early years of the Viking age. Later they undertook step-by-step strategies of conquest with well-trained soldiers.
But for all their trustworthiness as fearsome warriors, many more Vikings explored, traded and stayed peacefully in some other parts of Europe. Right after the first invasion, there is little indication of York being a warrior community - no warrior graves have been found in the city and very little weaponry.
Fishers and farmers, Vikings were a self-sufficient people who could take full benefits of what nature provided. Vikings fashioned intricate tools and were skilled craftsmen.
So why did the Vikings undertake hazardous sea adventures in longships to Britain and other areas of Europe? It may have been that the number of people increases allowed more men to leave their farmings and take a trip further afield. These explorations coincided with quick and significant improvements in boat developing.
An additional reason was to amass wealth. Many towns boasted remarkable assets, from farming produce and raw materials to precious church possessions like lavishly-decorated manuscripts. That made them lucrative targets.
Later numerous Vikings chose to settle in these areas, instead of return home with their loot. This is what occurred in and around York.