Q:

Can someone help me understand DIACs and TRIACs

These are something that has confused me for quite some time. Just like a lot of you, I'm sure, I'm a tinkerer. I like to take things apart, salvage parts, etc. A lot of stuff I take apart have a triac for AC control (a good example is a broken heater fan I'm trying to fix, but the triac isn't the issue there).
I know (or I think I know) that a triac is used for phase control of AC circuits, but I always see them used with a diac in schematics, and that's where the main confusion comes in. In almost everything I've taken apart with a triac in it, there's no specific component that comes up as a diac (when I look up part numbers).
My understanding is that a diac can simply be two regular schottky diodes in parallel with reverse polarities, is this true? I'm not overly fond of the idea of trying it with live AC without understanding it a little better. I'd like to phase-control a 120V resistive heating element (or more than one, if possible) and I just want to be more comfortable with what I'm doing. The internet keeps contradicting itself with the information I find.

A:

There are many diacs (I don't believe it should be capitalized) available on the market, most currently sold are variations on the DB3 part number. 
Older numbers included the ST2 originated by General Electric. 
A diac is a bidirectional switch that breaks over at a controlled voltage in each direction. It does not behave like two zener diodes- it has a negative resistance characteristic like a thyristor- the voltage across it drops (assuming the current is limited) and it will continue to conduct until the current drops below the holding current. 
A similar functionality is provided by a SIDAC device.
You could replace a diac with four BJTs (2 NPN/2 PNP) and a couple zeners and a couple resistors, but a couple ordinary diodes are not going to behave the same. 

