Passive insect eliminator. That is, it passively eliminates insects, not that it eliminates passive insects...specifically. Flying insects, mostly. *sigh*
In its essence, a bug zapper is an electrified light source. Flying insects are attracted by the light and, when they get close enough, are killed by the bug-lethal arcing of electricity. Bug zappers are mounted high above the ground and enclosed in a screen (or should be) so they don't become squirrel- or small child-zappers.
brzzzzzaappp!
The sound of a bug zapper is one of the sounds of summer. It fades into the background after a while, mixing in with the crickets, katydids and frogs. From time to time, it emerges from the cacophony with an especially loud and lengthy zapping. This indicates that some poor 'ol granddaddy of a moth or something is now taking tea with his insect Creator.
I find an ironic sort of humor in bug zappers. The bugs want to get to the light so badly. They orbit for hours, trying to find some way through the impeding screen. Then, when one occasionally reaches its goal, after all of its toil, it is obliterated. "This wonderful light is not at all what I expected!" It's kind of funny (hmmm...). Or, maybe I'm just a little sick.