Possible Duplicate:
Prevent gedit from creating files with the '~' suffix

I've noticed that every time i edit or in anyway modify a .txt, .php, .html, .doc etc file, after I save it, an extra file gets created that has the same name as the original file except that it has an extra '~' after its extension.

For example: 'index.html~' , 'name.php~'

What are these extra files? Is it safe to delete them? Is it possible to stop them from appearing at all?