HDR increases the Dynamic Range (High Dynamic Range) - the difference between details in the darkest parts of the picture and not washing out in the brightest parts. HDR+ is just a larger range. HDR+ Enhanced is a still larger range. (Basically the phone takes a range of pictures [it's called exposure bracketing], then a little chip [that Google designed - all other makes do it in software and it can take a few seconds or more between your ability to take shots] combines the underexposed bright parts and the overexposed dark parts into one picture. If you leave it set to Enhanced, it will almost always give you the best effects. About the only time I can think that it pays to "dial it back" [to HDR+, HDR or turn HDR completely off] is when you need to snap shots off as quickly as possible - like a series of a horse running, so you can capture that instant when all 4 feet are off the ground.)