You'll get better answers than mine, but my laymans terminology is this....

the pattern of a tartan is known as a sett

the sett is made up of a pattern of thread colors and counts for each color to make a line color narrow or wide.

The threads that go the width of the fabric are the weft and the threads that go the length of the fabric are the warp.

In tartans the warp and the weft are usually identical.

Here's a little sight I found that might offer some input

http://clanmckerrell.awardspace.com/tartan.htm