The Viking social structure was comprised of three main social classes: earls, free men (and women), and slaves.
The earls (Old Norse jarlar, singular jarl) were at the top of the social hierarchy. Traditionally – including much of the Viking Age – the earls were chieftains, warlords who had won great wealth and a following of loyal warriors through their successes in battle and raiding. Later in the Viking Age, when the first kings rose to power in Scandinavia, the earls became aristocrats who were subordinate to the kings and held land for them at their pleasure.
Most people in Viking society fell into the second social class: free men (Old Norse karlar, singular karl). Most free men were farmers. Some free farmers owned and worked their own land, while others labored for wealthier farmers, often in exchange for permission to farm a portion of their employer’s land. Other free men were craftsmen, merchants, or soldiers. Unlike slaves, who had essentially no legal standing, free men enjoyed the protection of the law.
Viking warriors primarily came from the class of free men. They tended to be young men who had little wealth to their names, and who went raiding in order to acquire some, whether in the form of allotted land to farm (in Scandinavia or in lands where the Vikings settled), or portable wealth like silver. They were generally unmarried, and so had no duties tying them down to a farm and domestic life in general. Inheritance customs in the Viking world typically meant that the older a son was, the more he could expect to inherit from his father, so Viking raiders were also disproportionately younger sons who hadn’t inherited much from their fathers. These ambitious young men were dissatisfied with their lot in their home countries and wanted to better it – and, crucially, since they were free and not slaves, they had the means to do so.
Slaves (Old Norse þrælar, singular þræll) made up the third and lowest stratum of Viking society. They were usually either used for farmwork or sold, sometimes within the owner’s home region and sometimes abroad in the slave trade that flourished throughout Europe and Asia at the time. Sometimes when a slave’s master died, the slave would be sacrificed and buried with him, perhaps to accompany him to the land of the dead and serve him in the afterlife.
There were three ways to become a slave in the Viking Age. The first was simply to be born to a slave, since the children of slaves were also slaves.
The second was to be captured in war. According to the ancient way of thinking, anyone who was captured in battle and whose life was spared had been given a tremendous gift, namely his life, which he had to pay back with an equally tremendous gift: his freedom. This reciprocity was the primary justification for slavery.
The third way to become a slave was to go bankrupt. The principle of reciprocity, of giving up one’s freedom in exchange for one’s life, applied here, too. An extremely poor but free person could give up his freedom to a better-off person in exchange for having his material needs taken care of. This was especially common when the poor person had gotten that way due to debts, in which case his freedom was the only thing he had to offer his creditor.