There are three separate types of optics that are found in 99% of all binoculars: Galilean, Porro, and Roof Prism optics. They all have their advantages and disadvantages, but Galilean optics are generally of lower quality and found in less expensive binoculars that are not designed for rugged use with the rest of your tactical gear. Porro prism binoculars are generally larger and produce brighter images than Roof Prism optics, but Porro prisms may also need to be recalibrated and set back into place at some point in time during their useful life. Roof prism optics are generally maintenance free.
Higher quality binoculars that will be useful in your bag of tactical gear will also include an anti-glare coating known as magnesium fluoride on their lenses. This not only increases the quality of the image that you see when you look through them, but it makes them easier on your eyes by not transmitting glare from the sun or other bright light sources.
Unless you want binoculars that are only good for viewing objects at one distance (a fixed focal length) you'll need to make sure they have a focus knob on them. High end binoculars will have independent focus knobs for each side, while less expensive models will have a dual adjust knob with a diopter adjustment to adjust for your off eye being slightly different than the one used to focus. This method is certainly more convenient and less tie consuming, but the advantage to separate focusing mechanism are obvious.
Do yourself a favor - if you're buying binoculars for frequent use, look into buying a pair with image stabilization. While heavier and more expensive, they do offer much greater magnification than non-VR models, and that means you tend to get greater benefit from them, especially if you're a heavy user and they don't find their way to the bottom of your bag of tactical gear too often.
