Mine is one my dentist took a mold for and had made; it's not a heat-and-fit like you can find in a pharmacy. The guard fits only on my top teeth but completely covers the contact surfaces (there's gotta be a name for that, just can't immediately find it). It's hard plastic. Because it was molded for me, it also stays in place by itself.

Wearing the nightguard took me a few days to get used to. Now it feels odd to sleep without it.

The one caveat is that this sort of guard is spendy, and your dental insurance likely won't cover it. I got a rare break: my insurance covered half. When I looked into a replacement -- this one is getting rather worn at 17 years of nightly use -- my current insurance won't cover any of the cost.