As the clip only trims feathers and not the wings themselves, it does not cause the bird major physical harm.
Since the feathers do not have nerve endings, the bird does not feel pain. Therefore, wing clipping is similar to a person having their nails clipped or their hair cut.
An appropriate wing clip will allow a bird to glide across the room without gaining altitude. If too few primaries are taken, the bird will be able to gain altitude; if too many primaries are taken, the bird will plummet like a rock. Those experienced with wing clipping will only take one or two feathers from each side at a time and see how well the bird can fly before taking more feathers, to avoid causing the bird to plummet and potentially be injured in doing so.
Many people who keep birds as pets prefer to clip their wings so that their birds will not crash into mirrors, walls, or windows, all of which are serious hazards to flighted birds: A hard crash can result in instant death.
Other people clip their birds' wings to minimize the chances of them flying away if they accidentally get outdoors. However, in the right conditions -- particularly windy days -- birds whose wings are clipped can still gain altitude and fly out of their owner's reach.
Another reason for clipping wings is to keep pet birds that are social in nature (such as parrots) more tame. When birds are unclipped, they typically have a greater sense of independence and can be less likely to socialize with their owners.
When a bird's wings are clipped, an equal number of feathers are taken from each wing. In ducks and chickens often just one wing is clipped (i.e. 10 primary feathers of one wing), they are not good fliers and to clip one wing is enough to unbalance their flight and keep them grounded. To keep a bird from being fully-flighted, wing clipping must be done after every moult, but it is important to only clip the wings when the feathers have finished growing and there is no blood in them. If the shaft resembles the ink tube of a ball point pen clipping can result in the death of the bird. Many people clip their birds' wings themselves, although it is a good idea to have an avian veterinarian, pet store employee, or breeder demonstrate the process first.