Thinning

Many plants benefit from thinning. By opening up the head or crown, you improve the aesthetics of the plant by showing the structure or framework. Thinning also reduces diseases on plants, like roses, that are susceptible to mildews and rust. The increased airflow through the plant alters the environment around the plant making it less hospitable to many diseases.
Thinning overhead trees allows for more light to reach the plants growing beneath which helps them to grow, bloom, and add beauty to your garden.
Thinning is not done to hedges or screens when they are used for privacy.

Tip Pruning

Tip pruning can be used to shorten a seasons worth of growth effectively slowing the growth rate of the plant. Removal of 1/2 to 3/4 of the new growth is not uncommon when trying to limit the size of a plant.
Tip pruning also can be used to promote bushiness in the plant. Removing the first few buds on each branch will promote branching which in turn will develop a thicker plant. This is typically done in late spring to early summer.

Corrective Pruning

This technique is used to prevent or remove growth that could possibly be the source of problems in the years to come. Branches that cross or rub against one another can open the plant up to disease infection. Remove one of the offending branches. Choose the weaker or least desirable branch for removal.
Branches with real narrow crotch angles should also be removed. These branches often split easily from the main truck and can become a liability in storms. A branch that forms a 91 degree angle to the trunk is ideal. Many plants do not naturally form wide crotch angles. Select the narrowest for removal.
Removal of small branches can be done at any time of the year. Removal of any branch larger in diameter than your thumb should be done during the dormant season.
Corrective pruning can also mean reshaping a plant to achieve the desired habit. This is most commonly done to multi-stemmed shrubs; by removing of the lower branches of the plant, you can form a multi-trunked tree.

Crown Reduction

This type of pruning is done to reduce the size of an overgrown plant. This drastic type of pruning should be avoided by combining the techniques listed above as the plant matures. When other options are not available, here is what you do. Shorten one third of the major branches (up to 3" in diameter) by one third of their length. It is best to do this over a two year period. Balance the removal of the branches throughout the canopy; avoid removing the branches from one side, then the other. This pruning should be done only in the dormant season.