HO Scale Coupler Types

Knuckle, Sergent, X2f, ect.

There's 5 "basic" styles of couplers prevalent in our hobby.  They are as follows:

Dummy
Dummy's can be shaped like knuckle couplers, the prototype styled, or in the case of Glatzl's, prototype, though compatible with Sergent.  Most are plastic, and have non-working knuckles, thus making them a dummy.

Extremely cheap, though not very useful for any sort of operation.  Usually used as a stand in.

Knuckle
These are also commonly referred to as Kadee, Kadee-compatible, or KD.  Kadee being the originating manufacturer of the style, and the most commonly used.  Brands use metal or plastic in the construction, metal being the preferred style, as they're more robust.  These come in various designs, but all operate similarly, using a tension device (spring, or plastic tab) to hold a movable knuckle closed, and a bent metal rod extending from the bottom (some say to simulate the air line??) that actuates the knuckle for uncoupling, and some sort of centering device on the shank.