These different approaches to sketching can be used to study the subject or as finished drawing styles.
A structural sketch defines the form of the subject through lines that emphasize accurate proportions and placement of features. By itself, a structural sketch has no values to communicate the lights and darks. Start a structural sketch by blocking in the overall form, then progressively adding smaller features.
A value sketch interprets the form of the subject through a range of lights and darks. The generalization of the values is the emphasis rather than the study of the placement of the features and proportions.
Also called continuous line sketch, a contour sketch defines the form of the subject through a continuous line. This technique develops your observational skills differently than the other sketching and drawing approaches because of its emphasis on shapes rather than values or proportional accuracy.
A black-and-white sketch, also called chiaroscuro, is a more extreme version of a value sketch and interprets the subject through highly contrasting values. The results of this approach can be bold and dramatic.