When Apple announced iOS 11 at WWDC this year, it promised the ability to take pictures using the HEIF format and videos in HEVC. These new formats allow your media to take up less space on your device and in iCloud, all while maintaining high quality.
There's one major problem though, and that's that HEIF and HEVC aren't compatible with everything just yet. For example, if you send an HEIF image to someone through the Skype app, they won't be able to receive it, even if they're also using an iOS 11 device.
And while this is all fine and good, it's important to be aware that this is the new default, so if you don't know about it, chances are that at some point, you'll try to send someone a picture and it won't work. The good news is that you can change it back.
All you've got to do is go to Settings -> Camera -> Most Compatible, and your iPhone will go back to saving media in the same way that it always has. If you ever want to go back to HEIF/HEVC, you can do that too, as there are most definitely benefits to both options.