Many people confuse pruning with shearing or trimming of plants.
There are a few techniques associated with pruning that are not actually considered pruning.
Lets break them down:
Shearing consists of hand propelled models or long gas or electrically driven flat blades mostly used on hedges or anywhere you want a sculpted tightly growing manicured looking plant.Shearing usually causes dense outer growth and occasional hand pruning is necessary to let in needed light to enhance the plants vigor
Sadly I have seen many trees and shrubs sheared when they would have been better suited for hand pruning to maintain their natural form.
Pruning is a much more complex way of shaping trees and shrubs to maintain their natural growth and shape and requires a different set of tools.
Usually hand Pruners, longer versions of hand pruners called Loppers, and a small hand saw designed to cut on the pull stroke.
A few reasons we prune are:
1)Maintaining the plants natural shape. The way mother nature intended it.
2)Thinning the plant to allow air movement through the foliage or (crown).
This helps the foliage dry faster after a rain or watering and decreases the chances of foliar diseases like Scab and Powdery Mildew.
3) Improves branching structure which enhances strength of branching.
4) Eliminates crossing, rubbing, and redundant branches.
5) The best time to prune is just prior to the growth season. We like to get out in late February because you wont incur new growth immediately and the trees are dormant which means no leaves in your way and less disease pressure.
Pruning is an art. There are many great Youtube videos and books written about it.
If you would like more information on this please consult one of these resources.