The Balanced Tree
If I had to sum up apple tree pruning in one word I would say BALANCE. In any one year, an apple tree should be making 3 things-new extension growth, fruit buds for next year, and fruit for this year. A young tree should be making more extension growth and little or no fruit (you may have to remove fruit to prioritise growth), while a mature tree will produce more fruit and less extension growth.
So how do you keep a tree in balance? By removing badly placed, crossing/rubbing, grossly asymmetrical, worn out or diseased wood in order to allow room for new growth to replace it. That's about it.