Well, everyone has their own definitions within the horse world. Whenever I hear someone refer to taking their horse out "hacking," I don't think of it in the same way as a real trail ride.
To me, "hacking" is like riding a horse down a wide, well-groomed dirt, gravel or grassy road or path. Or around the perimeter of a farm or through a field. They may just go at a relaxed walk with some trotting, or they may spend time doing flat-work exercises similar to what they'd do while riding in an arena. I think of "hacking" as a ride that lasts an hour or two.
Trail riding, to me, involves going "off the beaten path" a little more, longer time in the saddle and covering a bit more distance. The trails are more "natural" and trail-like with water crossings, natural obstacles, climbing and descending hills, navigating and picking their way through areas with tricky or slick footing.
Of course, it could all just be a matter of using different terminology, and that's fine as well. I'm not about to get into a discussion about who the "toughest" or "best" riders are, or which discipline is the "best," because those types of debates just go on forever and no one ever agrees anyway.
Please copy and paste this to your signature if you know someone, or have been affected by someone who needs a smack upside the head. Lets raise awareness.