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.
Velocity-sensitive drum pads allow you to modify the sound sample's pitch depending on how quickly you hit the pads. They provide a more natural audio experience, as the pitch depends on your movement, just like when playing the drums live.
The pads section is ideal to trigger loops and samples, using your finger drum skills.
When pads respond to the amount of force applied after initial impact they are pressure-sensitive. The pads can be used to drum rhythmic elements of a song or to trigger samples, but can also be used to play notes.