What exactly is engine lugging/bogging? Am I doing it?

I have both a car (1.7L I4) and a motorcycle (250cc V-twin), both with 5 speed, and I'm curious as to what specifically is lugging/bogging. Is throttle position/fuel flow relative to engine power output? Can it happen with Fuel-injection? Why is it bad? I'm very conservative driver, and normally don't need acceleration or power, so I tend to ride lower RPM. With my car, I notice it gets a little sluggish under 1600RPM, and while I tend to avoid going under that, sometimes I ride it out with a feather foot on the throttle just to keep my speed constant. The ride is still smooth, but the exhaust note (what little there is), gets a little deeper. In 5th gear, this happens when I'm about under about 32MPH or so. If I need to accelerate to get up to a higher speed, say 40-45MPH, I'll certainly do it in a lower gear, but otherwise I'll let it lazily pick up speed with a feather foot (say if I need to accelerate to 35MPH).

On a more extreme example, with my motorcycle (no tach), which is carbureted, I notice the exhaust note gets really deep when climbing steep hills or mountains, even at 50+MPH in 5th gear. Again, sluggish, but still smooth (no jerking or anything of that nature). Sometimes I downshift when I think I can't accelerate at all - If I can't accelerate by half throttle I'll downshift. But if I'm nearing the top of a climb I'll keep a steady throttle (1/3 - 1/2) take a speed reduction, and pick it up going back down. In that instance I'm a lugging or bogging?