A Line is, on a simple level, a mark connecting 2 points. However, how lines are used in an image is extremely important. By varying the quality of line used in an image, for instance from soft and controlled to harsh and dynamic the impact and content of the piece can change.
The 3 images below use very different values of linework to convey very different content. In the first image below the artist has used soft precise lines to accentuate the subjects cool, idealized features that recall the smooth balanced proportions of ancient marble sculpture. The next image uses quick gestural linework that invokes a feeling of intensity and a darting skipping rhythm. Whilst the third image uses harsh, angular lines that convey a feeling of manic anger and insane energy.