Definition of LINE
Line is a form that has length and width.  Line is a mark made by a tool and the usual art definition of line is a moving dot.  Line often suggests movement.  Line is usually the most dynamically forceful visual element in drawing

Line Weight
A line's weight refers to the width. Since a line's width is so tiny compared to its length we consider it to only have the later (length) dimension, but line is capable of infinite variety of weights.  

Character /Quality
Line can express an array of emotion and feelings.  A quiet flowing line can expresses calmness.  A graceful movement may reflect a feeling of dance or light-heartedness.  We often describe line as being solid and bold, quiet and flowing, delicate and dainty, jagged or nervous, angry, happy, free, excited, meandering....  Line also has texture, i.e. rough or smooth.

Line Direction  
Another important characteristic of line that we should remember is its direction.  A horizontal line implies quiet and repose.  A vertical line has more potential of activity.   A diagonal line most strongly suggests motion.

STRUCTURAL LINES
Structural marks: crosshatchings, hatching, stippling, washes -these are all optical values that help explain the subjects surface.  These structural markings give shape and dimension, direction, space and volume to whatever you are drawing.  (making something two dimensional appear to be three dimensional.)

Implied l (i) n (e) s
When two objects face each other the distance between is considered an implied line.

Contour Lines
Define or outline the figure or shape.