The numbers refer to the magnification and the diameter of the objective lenses. For example, in the specification "8x42" for a pair of binoculars, "8x" indicates the magnification, meaning that the viewed object will appear eight times closer than it would to the naked eye. The "42" refers to the diameter of the objective lenses in millimeters, which determines how much light the binoculars can gather for a brighter image. 

Yes. 

Monoculars have only one lens tube and are suitable for long-distance observation; binoculars have two lens tubes and are suitable for close observation. In addition, monocular telescopes are relatively more portable and easy to carry. 

High magnification can be less desirable because it tends to make hand tremors more noticeable, results in a smaller FOV, and amplifies earth processes such as heat being reflected. Higher magnification may also result in images that are not as clear and bright. Lastly, higher magnification makes it more difficult to initially find objects and to track moving objects. 

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