When viewing photos, webpages, email, or maps, you can zoom in and out. Pinch your fingers together or apart. For photos and webpages, you can double-tap (tap twice quickly) to zoom in, then double-tap again to zoom out. For maps, double-tap to zoom in and tap once with two fingers to zoom out.

zomming in or out by pinching

http://www.apple.com/iphone/tips/