Basic Techniques

Main approaches:

• Linear (line) drawing: subject constructed based on lines and outlines of shapes

• Tonal (value) drawing: subject constructed using gradations and changes of light to show differing planes and edges. tonal drawing also serves as a good bridge between line drawing and painting

Shading techniques:

• Hatching: using a series of parallel lines drawn in the same direction. can also be used to follow the contour of an object
• Cross-hatching: layers of hatch lines at different, overlapping angles
• Scribbling: shading using tiny circles 
• Stippling: shading using very small dots

Resources
http://learntodraw.org.uk/essentials/line-and-tone
http://www.learn-to-draw-lessons.com/drawing-pencil-sketches.html
http://www.mmwindowtoart.com/drawing/values&line.html
http://www.art-is-fun.com/drawing-techniques-creating-value-in-pen-and-ink-drawings.html
http://gurneyjourney.blogspot.com/2009/07/line-drawing.html (Picture 1)
http://www.explore-drawing-and-painting.com/value-drawing.html (Picture 2)