It's a tricky distinction.
Let's say someone insults you to your face, and insults your girlfriend you're with as well.
That was an action, and let's say you did nothing to trigger it.
Now, any action you take regarding the insults will be a reaction to the insults.
But, it will also be your choice of available reactions, which means
it will go on the karmic balance sheet.
I really don't think it's necessary to distinguish between them.
J.A. Appears to think there's an important difference, but
hasn't given an explanation.
If the two terms are literally synonymous regarding karma, there
would be no need to make the distinction in the first place.