They definitely weren't warships in the sense of possessing on-board ship weaponry of any kind. By that criterion, they were transport vessels. However, their shallow draft, speed, maneuverability (as light, heavily oar-powered ships) and relative stability made them excellent battle platforms, both for archers and for use in boarding actions.When fighting defensively, the ships could be tied together and effectively turned into a floating fortress, which is what Olav Tryggvasson did in the Battle of Svolder.I imagine fighting at sea also frequently gave the armies a lot more room to maneuver than land battles would have - considering the terrain of Norway in particular, which is almost universally mountainous, especially along the coast.

One factor which I'd argue played a major part in the cessation of Viking raids was the Christianization of the Scandinavian population. As Scandinavia was placed under the influence of the Roman Catholic Church, conducting raids against other parts of Christian Europe (and particularly against targets of religious significance such as monasteries) would have carried the risk of excommunication as well as secular ostracism for the perpetrators, which in turn would have been exploited by their political rivals. And rivals were never in shortage.In the case of Norway, the Christianization of the general population only began in earnest under king Olav I Tryggvasson, who reigned from 995 to 1000 AD. He didn't live long enough to finish the job, though, and worship of the old gods became legal again under Eirik H?konsson Ladejarl, who succeeded him as regent of Norway (Eirik was formally a vassal of king Svein Tjugeskjegg of Denmark, and never took the title of king himself, but he was the sovereign ruler of Norway in all but name).The practice of the old religion remained legal until the reign of king Olav II "The Holy" Haraldsson from 1015 AD onwards, during which it was illegalized again, this time for good. People who refused to comply were typically run out of the country or killed.The end of the "Viking Era" is usually approximated to around this time.