If you have a newer macbook, you really don't need to worry about whether its' audio or headphone. It acts as both. One of other reasons they call it audio (in addition to the reasons already mentioned above) is that the newer MB's audio jacks are also Optical. So they not only function as a headphone jack, but you can plug a fiber optic cable to it and plug it into more high end audio devices.
Yours has 2 jacks, one in and one out.
They can be called by either name:
Audio In/Mic (they are one and the same)
Audio Out/Headphone (they are one and the same)
They are also labeled via icons on them which is which so there's no confusing which is which same as any other laptop.