The big difference to me, is between ideas and story. I think there are no new ideas, but you can take an idea, a concept and make it your own. If I have an idea of a...I dunno, a cowboy falling in love with a city slicker or something, I can't claim that idea and say that no one else can write it because they are stealing my idea. But if I work to make it my own....flesh out the plot, develop the characters, write it my way, then I would say if someone took that, it would be plagiarism. It's my work at that point. Anything in between is a gray area, but most people are cognizant enough to know when something is theirs, and when it's not. If you don't put the work in, real work, not just changing a characters name or the title, then it's not yours and claiming it as such is wrong.