Comment by tmtvl

To expand on this, Emacs uses a system called "faces" to draw different textual elements, for example font-lock-comment-face to draw comments. Faces can have different attributes, like font family, foreground colour, background colour, having a box around it,...

So if I want to use a variable pitch font (like say... DejaVu Sans) to draw comments, I can set the font-lock-comment-face to use the "DejaVu Sans" family.