vectorges:
dhenry - Thanks for a nice concise explanation.CrossRoads - Thanks for nothing. When I googled, I got several seemingly conflicting pieces of information. One was that a triac is AC only. One was that a triac is isolated from the controller. One was that the triac was not isolated from the controller. ANd one was my original question. I was hoping that this group would have a nice concise answer from a group of intelligent electronics experimenters, which it did.
The problem with the internet in general is that there is a lot of misinformation out there, and if you are not an expert it may be hard to separate the wheat from the chaff. Fortunately this forum is able to do that. With a rare exception or two.
Instead of a general search on Google, try Wikipedia first. It isn't guaranteed correct, but in practice it is 99.99% of the time. It is the most important single site on the Web.
It appears to be a really well written article in this instance.
I don't have any TRIACs since I have not done anything with AC yet, but now I want some.... I am so freaking greedy it is hard to believe. ]