I touched on this in my last article, but today I am going to give a bit more information on canvas; what it is, and what it does.
Canvas is a relatively loose weave fabric that provides shape and stiffness to the jacket; without it, the jacket would be a floppy piece of fabric. Canvas in suits is typically a combination of wool or cotton, and some sort of stiffer animal hair, typically horse or camel hair.
In a perfect world, every jacket would be fully canvassed; unfortunately, canvas is expensive, and so most lower end jackets are fused. Fusing is a method of gluing a layer of interfacing to the outer fabric of the jacket; this makes the fabric unnaturally stiff, and the glue is prone to wearing off, causing the fabric to bubble.
Simply put, a fully canvassed suit offers the best form and drape; a fused suit is usually unnaturally stiff, and hangs awkwardly, bit it is cheaper; and the half-canvassed suit combines the both, it saves money on canvassing, while still providing moderate shaping to the chest area.
I have covered the basics, but if you want some more in-depth info, then here are some articles that you should check out:
Suiting 101: An Introduction to Suit jacket Construction by Black Lapel : An easy to read and informative article, providing the basic information.
The Art of Manliness Suit School: Part I – Fused vs. Canvassed Suits by The Art of Manliness : An in-depth, three part series of articles on suit construction, this article covers pretty much everything that you will need to know.