Summer is the time to remove vigorous growth and keep the tree to a manageable size. Winter is the time to structurally prune and to stimulate lots of new growth.
Say, for instance, you have a tree with 5 branches. In winter, the sap moves down out of the 5 branches. Then you cut 3 of the branches away. In spring, when the sap rises, it will be looking for those 3 branches, which aren't there, so the tree will send out lots of new growth. If, instead, you remove the branches in summer, you're removing the sap with the branch; therefore, won't stimulate as much vigorous growth next spring.
-When pruning, be sure to use clean tools. Apply bleach or some disinfectant to blades to kill fungi and bacteria.
-Be sure to cut correctly. Cut limbs to the collar, but not into it. Don't leave a stub to rot.
-Prune to a bud facing the direction you want a new limb to grow.
-Don't prune when rain is certain. Wet conditions are conducive to disease.
Also, with apple trees, its important to thin the fruit in the first 6 weeks of fruit formation or there won't be much fruit the following year. You should thin the fruit to 6 inches or so between apples.
Here is a video that will alleviate your fears of summer pruning fruit trees: http://www.youtube.com/watch?v=BZ6isBVPcpc
Another video concerning winter vs summer pruning: http://www.youtube.com/watch?v=WZQfbGlzz90
Some kinds of tree you don't want to cut on in certain months and in certain areas of the country (world). For instance I won't make cuts on pine trees when its warm enough for insects to be about for fear of the Southern Pine Borer, which could wipe out acres of pine trees. I hate to cut an oak in summer for fear of attracting insects. You should research the tree you want to prune and be aware of the insects in your area which could attack a wounded tree. Additionally, fruit trees are cheap and easy to replace. Large oak trees are not.