The D-Series phones provide a set of user definable keys. E.g.: D-100 and D-200 have 18 self labeling keys that are aligned in 3 layers to each side of the display.
The XT20 modules provide 60 self labeling keys, i.e. 20 keys on 3 layers.
When the user configures a key, he'll be presented with a list of key-functions. This list can be changed via provisioning, new key-functions can be added, others removed or changed. See Configuration templates.
You can discover the elements a single key-configuration is composed of here.