  Keller introduced a notion of quotient of a differential graded category
modulo a full differential graded subcategory which agrees with Verdier's
notion of quotient of a triangulated category modulo a triangulated
subcategory. This work is an attempt to further develop his theory.
  More than a half of the text is devoted to an overview of "well known"
definitions and results. As a result, the e-print is essentially
self-contained.
