  Notes of a 8h course given at the University of G\"oteborg during an Erasmus
exchange visit, June 11-15, 2018. It is intended for PhD and graduate students
familiar with $C^*$-algebras but not specializing in quantum groups. The
proofs, if included, are presented in detail, but most facts are not proved. A
special emphasis is put onto motivation of the theory and the intuition behind.
