I used to hear all kinds of horror stories about people who would show off or talk about their characters online and then having ideas stolen and shit like that. Made me waaay paranoid. XD
if you're concerned? do a poor man's copywright: iff you have something you want to develop into a story, send yourself a certified letter detailing the character/world explicitly.
It is! I learned it when I was like, fifteen. If you have a manuscript or anything, too, that can also be sent certified. Just as long as it's CERTIFIED, unopened, and clearly details the concept.