Screwgate - A carabiner with lockable gate, uses a threaded barrel to screw over the point of closure, preventing the gate from snapping over when loaded or shock-loaded. Often larger, stronger and heavier than simple snap gates, these are most often used with a belay device, for building anchors, holding slings in position and general rigging.
Magnetic clip - Essentialy the same idea as a screwgat but instead of a threaded barrel, the gate is locked by a magnetic clip that is released by pinching at the nose of the gate. Very secure, quick and easy to use and less likely to seize up.
Snapgate - These are the common or garden crabs you will find on most racks and quickdraws. Either Solid gate or Wire gate, the latter being lighter, the former stronger. Though these do not lock, they usually have some sort of hook or groove on the nose and gate to hold it in place when closed. Moat often used when clipping gear to the rope and racking.
Accessory - Many brands produce a lightweight carabiner that is only suitable for clipping accessories together etc. It is in no way suitable for load-bearing. These should always state, somewhere on it, 'Not Suitable For Climbing'.
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