An oval or D-shaped ring of metal with a
spring-loaded gate along one side of the long axis.
Annealed aluminum is usually used for lightness. However, where high resistance to
impact,
abrasion or heat is needed,
steel may be worth its extra weight.
The purpose of a carabiner is to connect rope, webbing, slings, harnesses, or other equipment together. Where an anchor is used to lift, belay or lower people or heavy gear, carabiners with a screwlock along the gate are used to ensure that twisting of the rope or webbing does not allow the gate to open accidentally. In the absence of locking 'biners, two regular carabiners can be used together with their gates on opposite sides.
Traditionally, a climber will carry a number of slings with carabiners at each end, hanging from her harness along with all the other gear. When a particularly safe (or dangerous!) point is reached in the ascent, some form of protection is inserted into the rock, ranging from a gun-fired bolt to a hammered piton to an asymmetric sliver of aluminum wedged into a crack. The climber fastens one 'biner of a sling to the protection, and the other 'biner to the rope. Now, should she fall, and assuming the protection holds, she will only fall twice the distance she has climbed from that point and not twice the distance to the last point.
Carabiners are amazingly strong. The Black Diamond Hotwire I carry my keys on will resist a force of 24 kilonewtons along its long axis and 7 across the short while weighing less than my keys, and still catch me from a fall to near-terminal velocity. Carabiner technology is constantly refined to increase strength and durability and reduce weight. However, all mechanical (as opposed to mental or organizational) climbing protection takes the form of a literal chain from the rock to you, and therefore you are vulnerable to failure of the weakest link. For this reason do not climb with 'biners (or other gear) that have been dropped onto a hard surface, subjected to extreme temperatures or chemicals, and certainly not with any gear whose history you do not know entirely.