75mm telemark bindings can be divided into three categories. You can find simple, inexpensive 3-pin bindings, bindings with cartridges on the cables to adjust the tension, or highly adjustable 'hammerhead' bindings with springs underfoot. 'Telemark' bindings are compatible with any 75mm tele boot, but not NTN-compatible boots.
NTN (New Telemark Norm) bindings are similar to alpine bindings in that they offer step-in convenience and a quick-release mechanism (but they are not DIN-certified). They also offer greater lateral rigidity, a plus for more aggressive skiers. These bindings are compatible with NTN boots only.
Both NTN and 75mm bindings may offer the ability to adjust the binding for easier walking or skinning. This feature isn't a necessity for inbounds skiing, but if you spend at least half of your time touring, it's a helpful addition.
This refers to the stiffness of the spring or cartridge in a 75mm tele binding. Heavier or more aggressive skiers will want a stiffer spring underfoot or larger cable cartridge for greater lateral support. Both of these mechanisms can be fine-tuned, with cable cartridges offering the advantage of being the easiest to adjust.