See The Traveler in History and Lore  for why aura can be used
Semblances and Quirks Edit
"Semblances and Quirks are special part of a wielder of aura you can't have a Quirk without a Semblance and you can't have a Semblance without a Quirk"- David Nikos, to Pyrrha.
As the quote says anyone with there aura unlocked will eventually develop a Semblance and Quirk they usally come about three days after each other your Semblance usally developing first.
Quirks are like Semblances but completely physical it can be a outward change that anyone can see or a inward change that you need a X-ray or surgery to see for example Ivorys Over Howl you wouldn't know about her quirk unless she used it or Lilys she actually been mistaken for a hybrid faunus becuase of her fangs, this also brings me to my other point some quirks have been mistaken for faunus triats but you can usally tell the difference.
Semblances are completely aura based if you run out of aura you can't use your semblance for example Volts Semblance Volt Store if he were to be depleted of aura all the electricity he stored would be gone. theres a better description of semblances on the RWBY wiki.