TRIAC

The difference is that scr is  usually involved with the DC side and the triac's witdh the AC side. Triac circuits can be used to control line-voltage (AC) powered devices. (this page about Triac, so if you want to see SCR page, please click on the SCR link on the right)

 the triac symbol show in below  its main terminals are designated as MT1 and MT2 .

A triac operates as the SCR but there is a diffrence in operation. 

the Triac can operate reverse and forward directions

characteristic curve for quick understanding of the triac given below and compare its curve to the SCR

the tiac can be triggered either a MINUS (-) or  PLUS (+) gate signal.

Some triac with good result;
 
     Q4006L4  (400 V / 8A) , Q6006LT  (600 V / 10A)

     L4004F31 (400 V/ 4A) ,  Q6010L5  (600 V/ 10A) 

     MAC15A   (400 V/ 15A) ,   NTE5610  (800 V/ 8A)