There are variations on this recipe from the Mediterranean to Indonesia, but it is essentially a wonderfully cool combination of cucumber and yoghurt.
The difference between cacik and tzatziki is in the yoghurt. Greeks favour a strained sheep's milk yoghurt, which like Camembert or Champagne, should have its own AOC, as there is nothing quite like it. In Turkey, where you will find cacik served throughout the meal, a natural runny yoghurt is used, which can be very refreshing. The Turks also produce an iced yoghurt and cucumber soup of the same name.
There have been olive pastes and concoctions around the mediterranean for centuries, but this dish is relatively recent. It was the invention of Chef Meynier of 'La Maison Dorée' in Marseilles and dates from the 1880s. My version uses a higher proportion of olives and includes lemon zest for added zing.