Pruning...

Tree surgery terminology

Crown Thinning: The removal of selected branches to allow increased light penetration and remove weight from the crown. Over thinning should be avoided, this will cause the tree to produce vigerous new growth (water sprouts).

Crown Reduction: The reduction of the crown, without 'lopping and topping'. Limbs are either cut back to their origin or to lateral branches which are capable of sustaining the remaining branch. Crown reductions maintain the structure and natural form of the tree. It is a time consuming process, but it is much better for the tree than 'topping'. Topping causes branch die back and vigerous new growth from where the cut was made, it shocks the tree, leaving the tree open to pests and diseases. It also lowers the amenity value of the tree, leaving the tree disfigured.

Crown Raise: The removal of lower limbs to allow clearance for buildings, cars, people. Excessive raising should be avoided.
Pollarding: This is a commonly mistaked proceedure. Pollarding should be started when the tree is young, it is a training system where the new growth is cut back yearly, keeping the size of the tree in check. It is NOT the removal of the crown down to stumps, this undertaken on mature trees which have not been pollarded in the past can be very damaging to the tree.