Trees can be pruned to help improve poor branching structure such as rubbing limbs or bad branch unions. They can also be pruned for clearance around structures and to reduce the height or length of limbs. Pruning can also be done to help achieve the view you desire.
Trimming or pruning can be accomplished at any time of year, except for fruit trees which should be pruned when dormant.
Trees should not be topped, which is a poor practice resulting in large wounds which may never heal and can lead to substantial decay.