What effect did the vikings have on medieval world?

The Vikings raided almost all of Europe at various points in time and their influence was seen in many places. They were masters of crafting weapons and building ships and designs of these was incorporated in numerous armies worldwide because of their usefulness. They were also master-crafters in metallurgy so their traders traded things like jewelry all over the world which also inspired many cultures. These are just some of the examples.

The answer is ethnic groups.