How do artists employ the various qualities of line in their works?

Question

Were I can find information on this

Answer 1

Here's a google search that may help:
http://www.google.com/search?q=qualities+of+line+in+art&rls=com.microsoft:en-us:IE-SearchBox&ie=UTF-8&oe=UTF-8&sourceid=ie7&rlz=1I7GGIE_en

Read several articles.

For one thing, the term is referring to the thickness or thinness of a line, whether it's made with pencil, pen, brush, palette, etc. Lots of variations.