When you by binoculars, you will see a set of numbers. The first number represents the magnification level of the binoculars whereas the second number is the lens size. Magnification is the measurement of how many times bigger an object will appear through the binoculars. Before you rush to buy the binoculars which have the largest magnification, you need to consider what you will be viewing. Binoculars with very high magnifications are best for still objects because, with moving objects, you will get some shakiness from your hands. For bird watching, the recommended magnification is 7 or 8x.
The lens size is crucial because it represents the amount of light which passes through the binoculars and to your eyes. In general, you will want a larger lens size because this lets more light through and you get a better image which is clear and bright. However, large lens sizes are heavier and physically larger. You will really have to weigh which is more important to you: image quality or practicality. Obviously, if you are going to trek to the mountaintops to view birds and scenery, you won’t want to lug around a 110 mm lens (which would probably be about 15 pounds!). But, if you are watching birds from a park or driving to your location, then it isn’t such a problem to have a large lens on your binoculars.