Summer pruning
To direct the growth by slowing the branches you do not want or to slow the development of a tree or branch, pruning should be done soon after seasonal growth is complete. The reason for the slowing effect is that you reduce the total leaf surface hence reducing the amount of food manufactured and sent to the roots. Summer pruning is also good for corrective purposes as defective limbs can be seen more easily.
Winter pruning
Pruning during winter is a very common practice as this is the dormancy period. This results to a vigorous burst of new growth in the spring and should be used if that is the desired effect. It is advisable to wait until the coldest part of the winter has passed. Some species like walnuts may “bleed” when the sap begins to flow but this is not harmful and will cease when the leaf falls of.