"Pain" is a general word for things that hurt. An "ache" is a specific kind of pain. "Aches" are often described as "dull pains." They last a relatively long time--many seconds, minutes, or more--and are often described in terms of pressure, "it feels like a band is tightening around my temples and forehead." The kind of headache you get when you take too big a bite of ice cream is definitely an "ache." The sensation you feel in your muscles when you've exercised too much as an "ache." The pain of a bee sting, a pin prick, or a pinch are examples of "bright" or "sharp" or "shooting" pains that would never be called "aches."

An "ache" is a persistent feeling of discomfort, like a headache. Example: After I ran a mile, my legs ached. "Pain" would be immediate discomfort, not persistent. Example: When James punched my arm, I felt great pain. You can also use them both in a sentence. Example: I felt such an aching pain in my back. This would mean that you feel a persistent amount of discomfort.

While the words 'pain' and 'ache' both have the same meaning, they are not always interchangeable. For instance, you could say "I feel pain in my back.", "I have a back ache.", or "I have back pain.". However, "I feel ache in my back.", or "I have back ache." would be incorrect in American English. It should be noted that the sentence "I have a back pain." can be acceptable, although some would debate whether it is grammatically sound.