I think it stands for "hyper text component." Essentially it's a way to assign event handlers through stylesheets. There are a number of ways to do it, but the simplest is by using the 'behavior' characteristic, for example :
.someclass {
behavior : url(behave.htc);
}
behave.htc would have a list of the events you want to attach to the 'someclass' elements, and a mappings to the javascript funtions that should handled by them.
There's another method that's more XMLish, where instead of using a style, you define a namespace, import an .htc file to describe that namespace, and define custom tags within that attached .htc. These tags can have events associated with their event handlers in a similar way.
check out
http://www.webreference.com/js/column64/index.html for a pretty good description of the second method, or msdn.microsoft.com for a description of the first. This is an IE 5+ technology, but there's a standard of some kind that's fueling it.