These terms are sometimes used interchangeably, but there are important differences to note.
Engraving is a technique that removes layers of materials away from a substrate. When you engrave a substrate, the top layer is vaporized, which often leaves a contrast. Etching is along those same lines, but usually results in a lighter mark. There’s not as much depth there. If etching something delicate, it barely touches that surface and you can barely feel the difference.
Marking is often associated with engraving metals or polymers. Metal is marked typically for identification, tracking, or branding. There’s usually a change in color, for example, marking a black serial number. Sometimes you may notice a color differentiation, but it’s more of a surface treatment.