Viking society was divided by classes (similar to medieval feudalism) but it was possible for people to move up in social class.
At the top of the hierarchy were kings and earls. Traditionally, kingship was passed down through inheritance but not always. Also, their power was bound by laws and customs. After the Viking period ended, earls became subordinate to kings. Icelandic Vikings did not have kings.
Beneath the kings and earls were aristocrats and jarls (regional chiefs). They provided military strength to their kingdoms. And they leased land to the farmers (similar to feudalism).
The karls or karlar made up the middle class which were further subdivided into well-off landowners and merchants and, further down the ladder, craftsmen and tenant farmers. When the kingdom was at war, they joined the army to defend their leaders. There were some men who became professional fighters. Karls who became wealthy could become jarls.
Slaves or thraell were at the bottom of the hierarchical chain. They could not own anything and were not paid to work, but they did keep tiny portions of profit when selling goods and could eventually pay for their freedom. Vikings often took prisoners during raids and either kept them for labor purposes or sold them at slave markets. People who committed crimes or who fell into debt could also be enslaved.
Picture References:
Cnut - History Channel - 6 Viking Leaders You Should Know
Slave Chains - National Museum of Ireland - Nine unmissable objects from Clontarf 1014
References:
National Museum of Ireland
Smithsonian Magazine - The Little-Known Role of Slavery in Viking Society
"Everyday Life in Viking Times" by Michael Gibson, ISBN#0-7500-1472-5
