What's the difference between asking questions here and starting discussions?

I know this may sound silly, bbut I just noticed there are two tabs on your screen you see when you click "Forums"- one says "Ask A Question" and "Start a Discussion". All this time I have been using "Ask a Question"- does it make difference which tab I use? I ask questions and most of the time they turn into discussions. I'm not nitpicking I just want to know for future reference. Please help.