There are several things separate a high quality set of binoculars from a lower quality set.
The better the quality glass used in the prisms, the better image quality you'll experience. The two most common types used in binoculars are BK7 and BAK4. Binoculars with BAK4 prisms are more expensive, but will give you sharper images. BK7 binoculars are cheaper, but the image will be more distorted, especially around the edges.
Binoculars filled with nitrogen instead of normal air prevent your binoculars from fogging up. You'll pay more for this feature, but you'll be glad you have it if you do bird watching in wet areas.
Manufactures often coat lenses to reduce glare, increase the amount of light coming in the binoculars, and improve the image quality. Look for the phrase "fully multicoated" if you want the highest quality.
Field of view is usually measured in degrees, or in width at a certain distance.
For example, a pair of binoculars might advertise a field of view of 8.5 degrees. Another pair might advertise a field of view or 400 ft at 1000 yards. This means that if what you are looking at is 1000 yards away, the image you see through the binoculars is 400 feet wide.
A larger field of view is usually more desirable for bird watching, as a larger field of view makes it easier to track moving objects.