You do not have to toss out an apple because it is brown or bruised. The fact is, a bruise is no different than the browning the occurs when an apple is sliced and exposed to air. Bruising crushes the cell structure, exposing cells to the air inside the apple. This reaction is called oxidation, causing the browning. Although, it might not be pretty, the bruised portion is safe and healthy to eat.
If you do not like the look of brown apple slices, cook them. Heat destroys the enzymes responsible for the oxidation process, thereby preventing the apples from browning