Velocity sensitive keys are just spring loaded keys that sound out louder the harder you hit them. If you "lightly" press a key, the sound will be quiet; the harder you press, the louder it gets.
Similar to a weighted action keys, but with less resistance and a slightly springier release, semi-weighted keys are popular with many players. If you don’t need realistic piano response, but also don’t care for spring-loaded synth actions, a semi-weighted keyboard might be ideal for you.
The amount of keys the controller has on the keyboard.
The pads section is ideal to trigger loops and samples, using your finger drum skills.
With a touchpad, users can control the device by moving their finger on a touch-sensitive surface.
Aftertouch keyboards are sensitive to the pressure applied to keys after the initial impact, and while the keys are being held down or sustained.