An ache is discomfort that continues for some time. It is usually associated with a specific part of the body, such as a headache, a stomachache, a toothache, an earache. After you exercise, the next day your muscles will probably ache. An ache is usually not extremely strong, so you can try to ignore it. Pain is usually stronger, more sudden, and more difficult to ignore. You would feel pain when you cut yourself or hit your head on something. If you exercise and you injure yourself – break a bone or tear a muscle – you would feel a sudden pain.

there is a difference between these two words: when a person cuts a finger for example - he at once feels sharp PAIN ACHE is used when the pain lasts long - like tooth ache...headache...back ache... E.g. When I hurt my arm, I felt a sharp pain, and now my arm aches.