Many iPhone applications let you view the screen in either portrait or landscape orientation. Rotate iPhone and the display rotates too, adjusting automatically to fit the new screen orientation.

You may prefer landscape for viewing webpages in Safari, or when entering text, for example. Webpages automatically scale to the wider screen in landscape, making the text and images larger. Similarly, the onscreen keyboard is larger in landscape, which may help increase your typing speed and accuracy.
The following applications support both portrait and landscape orientation:
Safari
Messages
Notes
Contacts
Stocks
iPod
Photos
Camera
Calculator
Movies viewed in iPod and YouTube appear only in landscape orientation. Street views in Maps also appear in landscape.