I would like to add to PikeWake's writeup that the "vikings" (when not plundering and pillaging) were also great
explorers and
settlers.
Iceland and
Greenland were settled by early
Norwegians, as well as the
Hebrides,
Orkneys and
Faeroes.
Leiv Eriksson even has a thorough description of the
Labrador coast; the land he called
Vinland (indeed, a viking settlement has been found in
L'Anse aux Meadows in
Newfoundland).
The influence on England was not only as invaders, but a great deal in trade and settlements. York is for example an old "viking" settlement.
Oh, and no viking ever had a horned helmet, that's a celtic thing... :)