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.