What is a Norway rat?

Are you talking about Rattus Norvegicus? (This is information from Wikipedia which you easily could have gotten yourself). It's often called called Brown Rat in English. It is the dominant rat in Europe and parts of North America, big and brown, and the most successfull mammal in the world next to humans. Charles Dickens think they gave it that name because "it is said to have been imported into this country (America) in a ship-load of timber from Norway. Against this hypothesis stands the fact that when the brown rat had become common in this country, it was unknown in Norway, although there was a small animal like a rat, but really a lemming, which made its home there." Check the web for more