What is HDR in the Camera app?

HDR - high dynamic range.

If you have that option on, and take a photo, you will get two photos one with HDR and one without it.

http://lifehacker.com/5991508/what-is-hdr-and-when-should-i-use-it-in-my-photos

HDR is High Dynamic Range. The iPhone will take three pictures with different exposures and layer them into one picture.

A high dynamic range image combines a series of photographs, each shot at a different exposure: underexposed where everything is darker, overexposed where it's lighter, and properly exposed in the middle. The best parts of these images are put together into one shot that brings out details in both the shadows and the highlights—information that would normally be lost in a single exposure though visible to the human eye.

Apple's version of HDR combines three images taken with the iPhone's back camera in quick succession. You can have your camera can save a copy of the regular photo in addition to the HDR image in your camera roll.

Settings > Photos & Camera > Keep Normal Photo

Works good when the camera (iPhone) and object are practical still. If fact the iPhone takes 2 pictures within a fragment of a second. One with normal exposure and one over or under exposed and then combine them to a "high dynamic range" photo. Because of the different time that pictures captured if your iPhone or the object are moving the result may be blurred.

Sorry for my English.

Thanks for the info! But actually I'm replying to say that your English is almost perfect, please don't feel the need to apologise for it!

i am simply responding to compliment your proficient command of the English language. I'm assuming from the apology it's your second (or quite possiblly 3rd or 4th) language. with that said, I know many people that grew up speaking English and still can't compose a sentence that would compare to your post.

In recognition of Cheonweb's response:

Your answer is very thorough and concise.

I wish I possessed the drive to learn how to transmit as clearly as you do in a second language. You rival the English language skills of many whom speak only English.