There's probably an official explanation from an experienced personal trainer, but since nobody else answered... I'm going to say that alot of it is relative, but here's a general "unofficial" guide:
Light: Simple, light, toning exercises. Stuff that older ladies would do. Nothing that could ever cause serious injury. Light weights and fairly high reps done on a casual basis.
Moderate: Regular lifting for the purpose of staying healthy and fit, yet not living in the gym... maybe 3-4 times a week. Rarely, if ever, maxing out. Typically the standard 3 sets of 8-12 reps per exercise. Lots of body weight exercises such as chin-ups, push-ups, sit-ups, etc. Typically stuff that's on the level of not having a serious need for a spotter.
Heavy: Lifting several times a week, maxing out constantly, leaving the gym lightheaded and hardly able to walk... serious straining with the risk of serious injury if you do something wrong.
