A thing is the governing assembly in Viking societies made up of the free men of the community.
In the pre-Christian clan-culture of Scandinavia the members of a clan were obliged to avenge injuries against their dead and mutilated relatives. A balancing structure was necessary to reduce tribal feuds and avoid social anarchy. We know from the North-Germanic cultures the balancing institution as the thing ("ting" or "žing"), although similar assemblies are reported also from other Germanic peoples.
The thing was the assembly of the free men of a hundred (härad/herred) or a similar district. Hierarchies of things could exists, so that the local things were represented at the thing for a larger area, for a province or land.
At the thing, disputes were solved and political decisions were made. The place for the thing was often also the place for public religious rites and for commerce.
The thing met at regular intervals, legislated, elected chieftains, and judged according to the law, memorized and recited by the "law speaker" (the judge). The thing's negotiations were presided over by the law speaker or the chieftain. In reality the thing was of course dominated by the most influential members of the community, the heads of clans and wealthy families, but in theory one-man one-vote was the rule.
Gotland, as an example, had in late medieval time twenty things, each represented at the island-thing (landsting) by its elected judge. (The judge also conducted the local thing.) New laws were decided at the landsting, which also took other decisions regarding the island as a whole. The landsting's authority was successively eroded after the island was occupied by the Teutonic Order in 1398, then sold to Eric of Pomerania and after 1449 ruled by Danish governors.
In late Swedish medieval time the thing-court consisted of twelve representatives for the farmers (free-holders or tenants). Still in the 20th century ting is the name of the lowest courts of justice: "Häradsting" and "Tingsrätt".