This tutorial explains several different ways to apply strokes when shading. It can be applied to drawing with pencil, markers, pens and other mediums including digital drawing.
For a practical and very basic application of the first four examples of these strokes you can see:
One Directional Strokes
Straight one directional strokes are the most basic shading technique.
Crosshatching is a shading technique where stroke is applied in two directions with overlapping strokes and is probably the most commonly used shading style.
Curved Stroke Shading
Curved shading strokes are useful for shading objects with a curved surface where you want to emphasize the shape of the object.
Crosshatch Curved Stroke Shading
Similar to the non-overlapping curved strokes the curved crosshatch stroke can be used for shading drawings of curved objects. The advantage of curved crosshatch strokes is that they tend to look more natural.
Smooth or blended pencil shading can be useful in certain cases where you do not want there to be a visible pencil stroke.
Circular Stroke Shading
Circular stroke shading is less common but can still be very useful for certain effects (smoke for example).
This tutorial went over some of the more common stroke types and shading techniques but you can always experiment with more if you feel it’s appropriate for what you are drawing.
For more applications for these strokes see: