You can view many of your iPhone apps in either portrait or landscape orientation. When your rotate your iPhone the display rotates too, adjusting to fit the new orientation.
How To Lock / Unlock Screen Orientation On iPhone
This tutorial works on iPhone 3GS, iPhone 4, iPhone 4S, and iPhone 5.
Lock Screen Orientation
If you do not want to use screen rotation then you can lock it. To lock the screen in portrait orientation, double-click the Home button, swipe the multitasking bar from left to right, then tap the orientation icon.
The orientation lock icon will appear in the status bar when the screen orientation is locked.
Unock Screen Orientation
To unlock the orientation you need double-click the Home button, swipe the multitasking bar from left to right, then tap the orientation lock icon.