GOLDEN RULES of pruning-
1) Study your tree and think about its needs
2) Keep the tree in balance
3) Don't overdo it (too little pruning is usually better than too much)
4) Don't let a jobbing gardener loose on a beautiful but neglected old tree
As one of my teachers at medical school said "Principles are all". If you understand how the trees grows and fruits, you will know WHY you are pruning and be more likely to get it right. Think about how the tree grows. First there is the main leader or trunk. Then there is the framework or scaffold of branches. From these branches arise laterals, and from the laterals sub-laterals, twigs if you please. From these, and indeed from the medium sized and smaller branches, arise fruiting spurs. These bear the fruit buds which produce blossom, and hopefully fruit. If you study a tree, you will see that fruiting buds and spurs normally arise from wood that is at least 2 years old. (in some years, some trees will form fruit buds or even spurs on the current year's wood, but this is the exception to the rule) This is fundamental-if you remove all the young wood by ignorant pruning, it will not mature into fruiting wood. One of the common errors in pruning apple trees is what I call the "pudding basin haircut" where the tree is clipped around to give a smooth outline, like a topiary hedge. This always reduces fruiting, and I find it quite ugly.
The normal growth of a leader
Let us consider and describe a single leader or lateral over three years, below.
Year 1, a slim pencil of straight new extension wood grows from the main trunk or a branch.
Year 2 the wood that grew last year develops some lateral spurs which develop fruit buds. From its end, a new 'pencil' similar to last year's growth extends. If you look at a tree, trace back from the tip of the latest extension and you can see a line around the leader where this year's wood begins and last year's ends.
Year 3 the wood that grew in year 1 produces its first blossom, and hopefully fruit. The year 2 wood produces fruit spurs and buds which will fruit next year, and the current year's growth extends the leader, another slim pencil of clean new wood as in year 1 above. See my first Youtube pruning video to follow this, but best of all have a look at a tree, tracing back from the tips, and work it out for yourself.
Year 4 as above, by now the spur systems of the year 1 wood are in their second year of fruiting and are probably getting a bit thick and touching nearby branches, leaders and fruit spurs. In other words, the tree is thickening up. If it hasn't been pruned yet, it will probably need attention now if optimum fruiting is to be maintained.
Obviously the above is a typical and as with all biological systems there is variation, for example in a good growing year, vigorous specimens of certain varieties can produce and ripen fruit buds from the current year's growth. It is also clear that the above process is limited by available nutrient, water and the effects of pests and diseases. The whole process slows down in a very old tree, which produces almost no new extension growth and has gnarled, ancient fruiting spurs. Again, some trees are more vigorous in old age than others (as the Psalm says "they bring forth fruit even in old age")
Additional information about pruning new trees is available from the RHS.