Brad has already linked to my article, which goes into greater detail than I can here. But just as a quick note, remember that it is perfectly permissible to weave a tartan in different hues or shades. It does not change the tartan itself. So terms like modern, ancient, weathered, muted, reproduction and the like all describe the same tartan woven in different hues.

On the other hand, terms like dress or hunting refer to different tartans entirely; as do other descriptors, such as the use of geographic locations. Stewart of Appin and Stewart of Fingask are tartans for different branches of the clan Stewart.

Of course there are exceptions to all this. For example, MacLean of Duart is not for a particular branch of the MacLeans, but for the whole clan, whose seat is at Duart.

You may also like to read my article on tartan colors.
http://www.albanach.org/colors.html