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Carabiners are inexpensive (between $5 and $20), strong (most are rated to hold at least 20 kN, about 2.2 tons), and versatile. Virtually every climbing technique uses carabiners.
There are many variations on the basic carabiner design. The carabiner shown above is a ``D'' because the ends have an asymmetric shape that tends to push the rope against the solid side, away from the weaker gate side. An older variant (below left), the oval, has no such asymmetry, and is not as strong. Another variant, the bent-gate (below right), has a curved gate that makes inserting a rope easier. However, the bent gate also makes it easier for the gate to work itself open, making it less safe than other varieties.
A carabiner is safe until its gate opens. To increase security, two carabiners can be used in tandem with their gates reversed (i.e., opening in opposite directions). It is less likely that something would cause both gates to open at once.
An alternative to a pair of carabiners, the locking carabiner (shown above) has an additional mechanism that makes it harder for the gate to open accidentally: a sheath that covers the gate and the outer C-shaped portion of the carabiner. This sheath either screws into place, or uses a spring to hold it in place.
Each variety is well-suited to certain applications.
The ``D'' is the most versatile, although it must sometimes be used in pairs for added security. The oval is used where its symmetry is desirable, typically on longer routes. The bent-gate is excellent for rapidly securing the rope, although it is slightly less safe. Locking carabiners are best when taking the extra time to attach them is not a problem