Projection, protrusion, protuberance, bulge all denote something which extends beyond a level or a normal outer surface.
Projection is applicable to anything that juts out, especially at a sharp angle.
Protrusion applies to something which is thrust out or which pushes out so that it seems an excrescence or deformity.
A protuberance swells or pushes out, often in rounded rather than angular form.
A bulge is a protuberance or expansion of a surface caused usually by pressure from within or below.