Hatching and Cross hatching
Shading is a way to add depth to a drawing. Showing the shadows and lighting gives 3-dimensional elements to the 2-dimensional paper. There are many types of shading and one of the basic ones is hatching. Hatching can also be used as just a way that the drawing is made up. For example in the slideshow below there's Van Gogh's Starry Night.
Hatching is just using lines at different lightness/darkness and different spaced intervals to show the transition from light to dark on an object.
Cross Hatching is using these same lines as in hatching but using criss-crossing them.
Contour lines are the lines that follow along the shape of the object, this is another step to adding depth and with hatching contour lines are used to apply the shading to fit the object.
Here I'll show various hatching and cross hatching examples.