Climbing equipment- locking and belaying karabiners
Locking karabiners are used in situations where the consequences of the karabiner becoming accidentally unclipped would be very serious. They are a locking link between the climber and their security - be it their belay, belay device or primary runners. Locking karabiners are also generally much larger and stronger than non-locking 'biners making them useful for high loading situations or where a large amount of material needs to be held.
'Lockers' can be split into the following types:
Small offset D (10mm body)
Small offest D
These are normal climbing karabiners featuring a screwgate collar. They are very light and compact, useful for connecting oneself to important runners (e.g. belays, crucial pieces on long run outs). Due to their small size they are not recommended as your primary screwgates as they can only hold a small amount of material (ropes, slings etc) before the gate becomes difficult to open.
Standard offset D (11mm or 12mm body)
Extremely strong but heavy, these are used for high load situations. For many climbers these larger 'biners have now been replaced by the lighter 10mm karabiner.
HMS or Pear-shaped Karabiner
These are the primary karabiners carried by climbers. They are very versatile; having a huge gate opening and usable internal capacity, and they are essential for proper use of modern belay devices. The HMS also allows you to use the Munter hitch (aka Italian hitch) for belaying or emergency rappelling. HMS 'biners are not recommended for high load situations as they are weaker than D-shaped karabiners as the force is not taken by the karabiner's spine.
This is the most common locking design, requiring the user to screw down a metal collar in order to stop the gate from opening. When buying screwgates always check that the gate cannot be over tightened when weighted, as this can lead to it jamming shut when un-weighted.
Less popular, these gates are locked shut the whole time and must be twisted in order to open the gate. Some climbers find these devices fiddly, and some designs are less than foolproof. Nevertheless if security is paramount, or you are absent-minded then a well-designed auto-locking 'krab' is worth considering.
For the majority of climbing situations, standard aluminium lockers work best, as they are light and cheap. If you enjoy a lot of top roping or abseiling, then consider steel karabiners as a far tougher and more durable investment.