It used to. Videos, photos, web pages — when you turned your iPhone sideways, the screen would rotate into landscape mode. But now it doesn't. Why?
Chances are you've accidentally locked your iPhone into portrait orientation. As of iOS 7, Apple moved this feature into the Control Center, which slides up from the bottom of your iPhone screen.
This makes it much quicker and easier to lock and unlock iPhone rotation, but it also makes it a lot easier to accidentally do so without intending to.
If you're expecting your iPhone to rotate, and it isn't, check your orientation lock.
For full instructions on how to do so, visit our FAQ "How do I lock (or unlock) rotation in iOS?".