Behold the vast number of karabiner shapes, colours, locking systems and where they all fit in. Here is an overview of the different shapes and types.
What are the different shapes for?
'D' shaped biners are very strong for their weight. This is because the shape directs the loads along the super strong spine. They are perfect for making the bulk of your rack, including quickdraws and anchor systems.
Also known as Pears, HMS karabiners direct load over a wider space, making them perfect for accommodating moving rope, knots and large devices. Carry one for belaying and if multi-pitch/trad climbing, an additional one for tying clove hitches to your harness when making rope anchors.
Essentially, these are HMS karabiners which have a built in system to hold them in the correct position whilst belaying. Some systems, such as DMM's Belay master also make it impossible to close the biner without the gate screwed up.
Oval karabiners direct the load symmetrically down their center point. They are great for pullies or devices with a wide attachment point. Some people also choose them for belaying as they have a wide loading area whichever way they are oriented, however you could consider using a belay specific karabiner with a captive gate.
Karabiners with integrated pullies
These biners have pulley wheel on the area of load, allowing the rope to run through with less friction making them perfect for hauling and rescue systems. Some people also use them in quick draws to decrease friction on the rope. They are great for decreasing drag on longer routes and reducing the force on either marginal gear whilst trad climbing or the rope if you are taking repeated falls on the crux of a sport route.
Non-locking - Snap gate
Snap gate is the general term given to karabiners with no locking mechanism. They are great for situations where you need to move rope in and out of the biner quickly, such as clipping quickdraws. They can also be used in cold conditions where gates may get clogged with snow or water and freeze up, where you'd double back two biners to prevent the rope from unclipping.
There are several locking options including screw lock, double/twist/quick lock and triple lock. By the names, it is obvious how they each work, via screwing or a twist and pull of the barrel.
Screw locks are the most commonly seen. They are simple, secure and the cheapest.
Double and triple action barrels offer a quicker and arguably more reliable locking mechanism by eliminating the chance of the gate being left open. This can be useful in most situations, as there will then be much less chance of the gate remaining open.
However, consider the balance between security and extra faff when adjusting knots or climbing with gloves.