I'm trying to create AC switch where I got BT134 instead of ACS108 triacs. Please help me to know the difference between the two.

 

The ACS108 has built-in protection agains excessive voltage applied across its main terminals; the BT134 doesn't.

 

The ACS108 has built-in protection agains excessive voltage applied across its main terminals; the BT134 doesn't.

So there is no problem replacing ACS108 with BT134 except that protection

 

If your load is purely resistive and the BT134 has generous voltage and current specs to suit your supply and load then there would be no problem.