When you ferment Apple Juice, you are talking about fermenting simple sugars that are giong to COMPLETELY ferment out if they are allowed to. That means that if you just let it sit in primary/secondary long enough, you end up with a slightly apple-y, dry drink with no sweetness at all.
When I say dry, think almost no mouthfeel, almost a "tart" or "puckery" mouthfeel. Same thing as a dry wine, you've fermented out all of the sugars and left no body. Some people like this style, most do not.
Lactose or artificial sweetener can be used to backsweeten, but it tastes "artificial" in the final product to me. Again, I venture to say some people are fine with artificially sweetened, most don't care for the taste.
Cider yeasts will advertise that they leave XXX% of the residual sugar behind, but I have found those claims to be hit or miss at best. Typically, left alone, you will end up with practically no sugar or sweetness in the final product.
The way to get a more natural BACKSWEETENING is taking your final, dry, unsweet product and mixing it with some unfermented apple juice to add back in that sweetness and more robust apple flavor. However, to do this, you must first neutalize the yeast so they don't just ferment out that sugar as well.
In a bottle, that requires either pasteurizing (Heating up each bottle to kill the yeast) or trying to "time" the fermentation and cold crashing when there are still remaining sugars, both of which are pretty involved and just a PITA in my opinion.
In a keg, however, you just keg your dry, unsweet cider, add about a 1/2 gallon of sweet apple juice directly to the keg (per 4.5 gallons of fermented juice), throw it in the kegorator at about 34-38 to hybernate the yeast, and you're good to go. These ciders taste more like the commercial versions (Woodchuck, Strongbow, etc.) with nice robust apple flavor and a good bit of residual sweetness.
Hope that is a good overview...if you want more details on either the keg or bottle method of backsweetening, let me know!