If you control the website that you are redirecting them to, then you can place a link on the site using custom URL which I describe in more details below. But if it's a site you don't control, you can have the user surf within your app using the UIWebView.
For an iOS app, you can create custom URL schemes that your app register with the system. Then on the web page you would create a link using that custom URL. That is how Apple launches the telephone.app or the mail.app from mobile safari.
For example: Let say your app is call BigBadApp. You custom URL would be: bigbadapp:// Now, you could create a link to your app would be: <a href="bigbadapp://launchedfromsafari">Launch BigBadApp</a>
You can pass any kind of information back to your app using the custom URL and your app will handle that information in the app delegate. For iOS 4.2 and later: application:openURL:sourceApplication:annotation:. The name of old delegate on earlier version of iOS is application:handleOpenURL:
For more information see check Apple Implementing Custom URL Schemes.
Also iOS developer:tips has a tutorial on Launching Your Own Application via a Custom URL Scheme.
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