For AC it will conduct in both direction on triggering pulse.
NOW
if we apply DC; suppose with A1(+) and A2(-) and apply a triggering pulse at gate then current will start flowing through it in only one direction, from + to - continuously.
so we can conclude that it will work on DC. you can do experiment on it.
Yes, Can be used in GATE terminal to trigger.. but once triggered it will continue indefinitely switched on. So it has very limited applications on the dc field.
Ultimately, The TRIAC is a three-terminal electronic device, which works as a switch for AC signals. With a small current injected on the GATE(can also be a DC source), it enables a relatively high AC current between T1 and T2 terminals.
If you want to only a swich DC voltage then the you don't need to go for Traic or SCR....you can simply use transistor or MOSFET for that purpose..... If you want to have contolled AC to DC retification then only SCR of traic are used .......
use simple and General purpose Commonly available transintors C828 or c1383 or 2N2222, for low power switching applications. OR D313 for switching high power applications. you can see data sheets to chose one among these according to your requirments.