Dashain or Vijaya Dashami or Dashera or Dussehra is commonly known as Durga Pooja. Whatever we pronounced it, but the main motive to say so is to recall Durga maa or Durga Mata.
We celebrate this because of the remembrance of the victory of good over evil. In Sanskrit, ‘dash’ means ten. Hence, this is the festival which is to be celebrating ten days.