What you're looking for is a WYSIWYG editor that is embedable in your backend.
WYSIWYG is an acronym for "What You See Is What You Get". In computing, a WYSIWYG editor is a system in which content (text and graphics) displayed onscreen during editing appears in a form closely corresponding to its appearance when printed or displayed as a finished product, which might be a printed document, web page, or slide presentation.
This way you can adapt a textarea to make it something like the editor you have on SO. Try googling "embedable WYSIWYG editor": http://tinyurl.com/nd3to4c
You'll find tons of javascript plugins that give you these features.