Whether you favor sweet cherries for simple desserts, or tart cherries for preserves or pies, cherry season is one of the hallmarks of high summer. Their stone-hard pits are only a minor inconvenience when you're eating the fruit out of hand, but they must be removed for the sake of your diners' dental work if you plan to bake with the fresh fruit. It's easiest to remove the pits with a specialized pitting device, but you can improvise if you don't have one.
Most department stores and kitchenwares stores sell cherry or olive pitters, which usually bear a vague familial resemblance to a stapler. Place your cherry on the pitter's concave platform; squeeze the lever down; and a rod presses the pit from your fruit. It's a messy process, so you might wish to wear disposable gloves to keep your fingers from staining. If you pit a lot of cherries you might wish to invest in higher-volume pitters, which feed the cherries down a chute to the pitting surface. Pressing down on a plunger pits the cherry, and ejects the hollowed fruit into your bowl.
You can also use a regular paring knife to pit the cherries, though the end result is cherry halves rather than a neatly pitted fruit. Cut them in half around the equator, then twist the halves in opposite directions. One half will separate from the pit; then use the tip of your knife to pry the pit from the other half. It's time-consuming, and you might find your knife-wielding skills impaired by the gloves, but it does work. Alternatively, a pastry-bag tip -- especially the "Bismarck" tip used for filling doughnuts -- makes a functional cherry pitter. Simply push it through the middle of the cherry, and it will force your pit out the other side.