Apple

A UISegmentedControl object is a horizontal control made of multiple segments, each segment functioning as a discrete button. A segmented control affords a compact means to group together a number of controls.

A segmented control can display a title (an NSString object) or an image (UIImage object). The UISegmentedControl object automatically resizes segments to fit proportionally within their superview unless they have a specific width set. When you add and remove segments, you can request that the action be animated with sliding and fading effects.

You register the target-action methods for a segmented control using the UIControlEventValueChanged constant as shown below.

[segmentedControl addTarget:self

                     action:@selector(action:)

           forControlEvents:UIControlEventValueChanged];

How you configure a segmented control can affect its display behavior:

If you set a segmented control to have a momentary style, a segment doesn’t show itself as selected (blue background) when the user touches it. The disclosure button is always momentary and doesn’t affect the actual selection.
Prior to iOS 3.0, if a segmented control has only two segments, then it behaves like a switch—tapping the currently-selected segment causes the other segment to be selected. (On iOS 3.0 and later, tapping the currently-selected segment does not cause the other segment to be selected.)

References:
	>How do I set up image in segmented controller? (From: Brian Christmas <email@hidden>)