A belt is simply a loop of tough, flexible material such as rubber and kevlar with a series of teeth that mesh with the teeth on specially designed pulley wheels to synchronize two (or more) rotating shafts. Belts are a cheap options for shafts that are not axially aligned and tend to run quietly and smoothly. Belts are typically tensioned either by adjusting center to center distance between pulleys or via a tensioner/idler pulley. This leads to very low backlash and a high efficiency (as high as 98%).
We have two types of belts available, GT2 (6mm wide) and GT5 (15mm wide). Each comes in loops of varying circumferences. The GT2 belts are rated for 6lbs of working tension while the GT5 are rated for 100lbs. Belt systems are fairly simple, you have 2 or more pulleys and a belt that goes around all of them. One of the pulleys is driven by your motor and the other(s) are either connected to your load or in the case of an idler pulley are simply free spinning to act as a tensioner for the belt.