As their name implies, locking carabiners have a lock of some kind on the gate which insures that it will not open even under stress. The simple way to classify them is to group them by the number of motions (actions) required to lock the gate: Screw (one-action), Double-action and Triple-action. The more actions, the more secure but the more actions (and hands that need to be free, not always easy when climbing) needed to lock the carabiner.
Below is a picture and description of 3 types of locking carabiners. Take a look, then answer the questions.