i always think of cider at "thicker"---seems to have a little sediment in it?? where as most apple juice seems, aside from the color, as clear as water.
I think that cider is usually processed from early picked apples, it usually has a sharper taste to it. Also cider (not the stuff that is mass marketed- the real stuff) is unpasteurized and so can then be used as the base for a Hard Apple Cider if wanted. Cider is also often pressed or squeezed out.
Apple juice is ofter pasteurized and I believe that steam juicing is often used (not sure) so it is essentially a "cooked" product where as cider would not be. Any "cider" you buy from the store is going to be glorified apple juice unless it has the label "Spiced Cider". Then it is going to be glorified spiced apple juice. I actually really like some of the "Ciders" that you can buy in the store- but after having the "real stuff" from MaryJanesFarm last year- I am a convert to pressed cider!
Both apple juice and cider are usually pasturized if you buy them from the grocery but you can find unpasturized if you have a local orchard. The BIGGEST difference in the two is the filtering process. Cider is typically filtered once using a coarse filter which still allows some of the apple particles to get through. This is why you can often see "stuff" floating around in cider and it needs to be shaken or the particles settle to the bottom.
Juice is often filtered several times called "clarification" by passing it through fine filters. This removes most/all of the apple particles making it "clear" and lighter in color.
The difference between cider and juice has nothing to do with spices or seasoning. 99% of cider you buy will be strait juice from the apple. MAny people do choose to spice it at home but this is a home process and has nothing to do with the actuall making of cider.
I used to work at an apple orchard that bordered our property every fall.