“Porro” or “roof prism” design: A binocular is actually two identical telescopes (with prisms added to correct the image) fit together in perfect harmony to be used by both eyes, producing one unified image.
All binoculars either use “porro prisms” or “roof prisms”. The ones using porro prisms, named after the inventor, Ignazio Porro, are the traditional design where the eye pieces are off-center to the barrels (zig-zag). This design inevitably makes the instrument more bulky and heavy, but in the case of compact porro prism binoculars the zig-zag design is reversed with the eyepieces further away to accommodate the distance between the eyes (interpupillary distance) and the objective lenses closer: The “reverse porro prism“. Example:
In the case of the ones using roof prisms the design is sleek, since the eye pieces are in line with the objective. Example: Nikon 7294 Monarch ATB 8×42 Binocular:
One definite advantage of the roof prism design is the fact that these binoculars can fold back quite dramatically when stored, which is especially handy in the case of pocket binoculars, where you want the binocular small enough to fit into your pocket. In most cases the hinges of pocket binoculars are either off-center or double to making folding back even more substantially. Example: Swarovski Optiks Pocket Binocular (8x20B-P, Black)
Porro prism binoculars are less expensive than roof prism binoculars for the same image quality, whereas roof prism binoculars are more compact and lighter, but are usually more expensive due to the more complicated manufacturing process.