a DIAC is a voltage controlled bilateral device and is triggered into conduction when the voltage across it goes beyond its breakover voltage. it is commonly used in phase control to generate gate pulses to a TRIAC or SCR,.
furthermore DIACs gives a near perfect symetrical trigger pulses at the positive and negative breakover voltage points to minimize the DC component in the load.
some of the uses are often used as gate triggers in the light controls, dimmers, power pulse circuits, voltage references inAC power circuits and TRIAC triggers for motor speed controls etc...
there is another device that incorporates the DIAC & TRIAC into one package, that is the QUADRAC.