Straight gate carabiners are the most common type that is currently available. They open, and allow the climber to easily detach the device from whatever ropes that it is currently attached to. They are the easiest, cheapest, and strongest type that is currently available. They have also been used for the longest, and are some of the most trusted types of climbing carabiners that are available.
Bent gate carabiners are just as strong as the straight gate carabiners. They allow things to be clipped in much more easily, as they do not lock. They are also much easier to unclasp. It is recommended that more experienced climbers use these, as they can be easy to detach by accident if you are not paying attention to what you are doing.
The locking carabiners are also very popular. They tend to be the safest, as the climber really has to go out of their way to get these undone. There are different kinds of locking carabiners that are currently available. Some require you to twist a knob in order to get the carabiner off from whatever it is attached to, while others may only require that you push a safety button. These are often given to new climbers who are feeling unsure about the task at hand and would like a little more peace of mind.
Keep in mind that whatever climbing carabiner you decide to purchase, you have to be sure of the quality, with Petzl being one of the most popular and reputable brands.Always make sure that the locking carabiner that you purchase has been certified. You may also want to comparison shop online, as the price of different types can vary greatly from one store to another.