Well, if you want to use a computer analogue (that has the consequence of oversimplifying things) sensory neurons are inputs, motor neurons are outputs, and relay neurons are computational and conducting. In reality, they're all a bit computational though. The sensory neurons can desensitize and can be modulated by genetic and electrophysiological processes - so too can the motor neurons. Similarly, there are internal neurons that only serve to pass the information, preserving it's quality, not changing or modulating it.