The Tsimshianic languages are spoken on the northwest coast and in adjacent areas of the interior. Gitksan and Nisga'a are so similiar that they are considered to be dialects of the same language by most linguists. As a result, in the literature they are often referred to together as Nass-Gitksan. However, the Nisga'a and Gitksan people are politically quite separate and for political reasons prefer to refer to Nisga'a and Gitksan as separate languages. (A comparable situation in Europe would be Dutch and Flemish, which are the same language. Flemish is simply the variety of Dutch spoken in Belgium.)
South Tsimshian is spoken at Klemtu and may represent the original language of Klemtu. It has been almost completely replaced by Heiltsuk, and increasingly, English.
The name of the language family is not entirely uncontroversial. Gitksan and, especially, Nisga'a people do not care for the name "Tsimshianic" because it seems to give priority to Coast Tsimshian. Nisga'a people sometimes refer to "Nisga'a and the related languages". However, we seem to be stuck with "Tsimshianic" since there is no generally accepted replacement that does not favor one language over the others.
The Tsimshianic languages are spoken almost entirely in British Columbia, but Coast Tsimshian is also spoken at New Metlakatla in Alaska. New Metlakatla was established in the nineteenth century by Christians who moved away in order to create a community in which they could live according to their new beliefs without conflicting influences.
The Tsimshianic languages are not generally considered to be related to any other languages, but there is a long-standing proposal that they belong to a large group of languages spoken along the Pacific Coast from California northward known as the Penutian language family. The status of the Penutian family and whether Tsimshianic is part of it is a matter of research and debate.
(text copied from http://www.ydli.org/bcother/bclist.htm#sal )