This is a quick tutorial/explanation I gave over on /loomis/
So there are two primary approaches to academic drawing. One is based on the objective structure of a subject while the other is based on the visual impression of what you're drawing. While they both may sounds similar because your visual impression is that of a structure/object but the way you go about representing it can be very different.
So we will start with the easiest approach to understand (the drawing to the right). This approach os often referred to as line drawing but it has more to do with form and structure. What this approach is meant to do is to represent the 3D structure of the object. We look for forms and try to represent them with lines. There's not much more to it than that. A contour line of a form is made to represent its edge and you emphasise overlaps and structural form where possible. This approach is very useful for when you want to gain an understanding of your subject and becomes very useful in the study of anatomy and perspective. Figure drawings in this approach aims to explore what the body is doing, the way it is behaving. Light and shade is not necessarily excluded but should be added in a manner that seeks to further describe the forms of the subject.
The second approach (drawing to the left) is often referred to as mass drawing. In this approach you do not consider the structure of your subject but rather what the visual impression of it is (the shapes of value/color hitting your retina). This approach relies on abstracting down your subject into shapes of value and to then be represented as an honest depiction of what you are seeing. In the drawing I have only represented the light and shadow shape of the subject and it is a fair depiction of what we would see if squinting at the subject. This approach is very useful for when you want to make a visual representation of what you are seeing rather than a structural one. You seek to explain what something actually looks like rather than what it might be. This is a less "intellectual" approach because you do not seek to explain anything more than what appears to you.
A notable difference between theses two approaches is the way light is treated. In the structural/line drawing approach it is often treated as a secondary element, used to describe more of the subject's forms. In the mass drawing approach, light and dark is everything. It is purely based on the shapes of light and dark and any structure/form in the drawing is purely a result of what appears visually.
When working with mass drawings you will often encounter a situation where the edge of a form is the same value as the background. Now there are two ways to treat this issue. If you were to work purely visually, you would not indicate that edge at all because it is not part of the visual impression. Another thing you could do would be to move away from mass drawing and define the edge even if it's not visually apparent.
Now both of these approaches are very useful there are gradients between them. It is wise to consider what most appropriate for the drawing you intend on doing before starting on it. I personally prefer mass drawings but line drawing is very important to learn about forms. One approach a lot of artist take is to first do a line drawing and then add visual elements over it so they get a best of both worlds.
Anyway, hope this helps some of you and if you have questions in regards to these two approaches or academic drawing in general, I'll try to get to them as soon as I see them.