You should choose your carabiner based on 3 criteria: use, shape and locking system
They are mainly found as a component of quickdraws or camming devices and will hold a climber in the event of a fall.
They are equipped with a screw to lock the gate of the carabiner against the body, which prevents the gate from opening randomly. These carabiners are used for important connections such as anchor points, belay devices and systems, abseiling, safety lanyards...
Carabiners come in different shapes depending on their use
D-shaped carabiners can be used for clipping in (at the end of sling) or for belaying (only with a Grigri). Compared with pear-shaped models of the same diameter, D-shaped carabiners are stronger as the load is transferred along the major axis of the carabiner.
Pear-Shaped (or HMS) Carabiner
HMS carabiners can be used for belaying (with all kinds of belay devices or an Italian or Munter hitch) and for clipping in , using a clove hitch. Their shape means the gate can open wider than other carabiners, wich makes them easier to use.
There are two kinds of locking systems for locking carabiners
This is the classical closure system and comprises a ring on a threaded gate
The carabiner can be opened with two or three (depending on the model) movements of the gate. The main advantage is that the gate closes automatically, so there is no risk of forgetting to lock it.