With so many different key mechanisms used, it can be confusing knowing what they all mean.
A traditional acoustic piano has 88 keys, all of which are fully weighted. With the exception of the NU1, all of Yamaha's digital pianos base their key mechanisms on that used by a grand piano (where the hammers come from under the string rather than in front). Obviously, the closer you can get to 'the real thing', the better.
Semi-weight keys
These offer some resistance when you play (unlike a keyboard) but are not fully weighted. One advantage of this is that the instrument itself is very light and portable.
Graded Hammer Standard (GHS)
The first 'fully weighted' piano key that Yamaha offer and preferred by teachers as it allows you to develop your playing technique. The weight of the keys are graded from heavy at the bottom to light at the top, reflecting the weight of a traditional piano key.
Graded Hammer (GH)
Provides greater accuracy for faster repetetive passages required by intermediate pianists. Professional grade components provide a smoother, quieter mechanism when striking the keys.
Graded Hammer 3 (GH3)
For many people this is actually regarded as the first 'proper' piano key. The reason is the keys return faster than GH and GHS keys. With the addition of a damper sensor, the GH3 mechanism allows the player to feel the weight of the key when depressed a second time, even when the player has not completely released the key after the first push of the key, and the note will still sound. This allows for quicker repetitive playing.

Graded Hammer 3X (GH3X)
The latest development to the GH3 system is the addition of an 'escapement mechanism'. This reproduces a slight clicking sensation near the bottom of the key travel. It has been designed in such a way that the click is discernible only on the lightest keystrokes, similar to the keyboard of a grand piano.
Natural Wood (NW) and Natural Wood X (NWX)
Natural Wood keys are created using Yamaha's unrivalled expertise of wood acquired through more than a century of crafting fine pianos. Just as with grand pianos, the material used for NW keyboards is cut from wood that has been carefully dried specifically for use in making musical instruments. NWX also features the escapement mechanism for enhanced subtlty and realism.
Linear Graded Hammer Action
Rather than changing the weight in different zones across the keyboard, the Linear Graded hammer action has a different weight for each key, enhancing further the subtle change in response a top-level pianist would expect.
Synthetic Ivory Keytops
Synthetic Ivory key tops reproduce the tactile surface of the Ivory keyboards once used in acoustic pianos. It features highly absorbent material that prevents fingers from slipping even after hours of performing, while retaining the ideal texture and feel.