Winter pruning involves trimming unwanted branches off a tree during the cold season, when trees are dormant and pruning isn't harmful to the tree. One reason for pruning is to remove diseased or dead wood from the plant, as the disease is likely to spread to the healthy parts.
In addition to the tree's dormancy, winter offers a number of other advantages for pruning. For example, trees without leaves show their branching structure more clearly. The wintertime is also less likely to facilitate disease transmission among plants, and insects are less likely to investigate the fresh wounds from pruning.
Winter pruning should have a clear purpose. With deciduous trees, this should include developing a natural style. It's not sound to shear tall shrubs, to top them so that they are shorter or to limb up a low-branching tree. The tree's natural outline should remain in place. The best pruning job looks neat, but as if no one has pruned the tree at all.
In addition to taking off diseased or dead wood pieces, gardeners prune water sprouts (stems growing at right angles from the branches) and suckers (straight stems growing from the base of the tree). Neither of these types of new growth become limbs that look natural or branch well. When pruning, the process should go back to the nearest branch or bud rather than leaving an open end or a stub.