Kriek is a gueuze beer with cherries or morello cherries. Different fruity gueuzes were produced later, but the traditional recipe was Kriek.
In the past, 'Krieken' (or Schaarbeek cherries) were used to soak with the lambic mixture that will be aged and blended to make the gueuze.
Schaarbeek cherries are in fact morello cherries with very little flesh. This is also why they were used, so there was almost nothing to chew.
Contrary to popular belief, Kriek beers are not sweet beers but acidic beers that manufacturers softened in the 20th century so that they would appeal to more people.