An ache is a persistent discomfort, typically dull so that you can try to ignore it, but sometimes all-encompassing, yet not sharp enough to describe as pain. Your legs would ache after a tough run; you would not describe this as pain. You usually get a headache, not a head pain. You would suffer pain when you cut your finger, then experience an ache as the wound heals. A pain is something more localised, often (but not always) short-lived, and something you'd be less able to ignore. Our vertebral column doesn't twist on the inside, therefore it is called as pain, not ache. You feel uncomfortable all over your body when you have an ache, but pain does not result in total body tiredness.