Deep Cycle batteries are designed to be more deeply discharged than starting batteries. They provide a continuous power over a longer period of time where a starting battery provides a short burst of energy to start your engine.
Deep cycle batteries have thicker plates that allow the battery to be discharged and recharged over and over. The shallower the depth of discharge, the more cycles the battery will provide. Manufacturers differ in the cycles that a battery will provide, but ideally if your battery is not discharged below 50% it will last significantly longer.
After you use a deep cycle battery, you should always charge it back up. To get the longest life out of a deep cycle, you should, whenever possible, start using the battery from a fully charged state. This will help you to keep your depth of discharge as shallow as possible and help to provide the most cycles out of your battery.