What's a redirect URI? how does it apply to iOS app for OAuth2.0?

Beginner programmer here, please pardon ignorance & explanations will be really nice :)

I've tried to read the tutorials for a certain OAuth 2.0 service, but I don't understand this redirect URI... in my particular context, let's say I'm trying to build an iPhone app that uses OAuth 2.0 for some service. I have an App ID that was generated, but i need to provide some sort of redirect URI to generate the API key.

Is this a URL that I'm supposed to host somewhere myself?? As the name suggests, I would think that the redirect URL is supposed to "redirect" someone somewhere. My only guess is that it's the URL a user is redirected to after they log in to the service.

However, even if that assumption is correct, I don't understand one other thing - how can my app be opened again after I've sent them to the browser for the user login?

Read this:

http://www.quora.com/OAuth-2-0/How-does-OAuth-2-0-work

or an even simpler but quick explanation:

http://agileanswer.blogspot.se/2012/08/oauth-20-for-my-ninth-grader.html

The redirect URI is the callback entry point of the app. Think about how OAuth for Facebook works - after end user accepts permissions, "something" has to be called by Facebook to get back to the app, and that "something" is the redirect URI. Furthermore, the redirect URI should be different than the initial entry point of the app.

The other key point to this puzzle is that you could launch your app from a URL given to a webview. To do this, i simply followed the guide on here:

http://iosdevelopertips.com/cocoa/launching-your-own-application-via-a-custom-url-scheme.html

and

http://inchoo.net/mobile-development/iphone-development/launching-application-via-url-scheme/

note: on those last 2 links, "http://" works in opening mobile safari but "tel://" doesn't work in simulator

in the first app, I call

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"secondApp://"]];

In my second app, I register "secondApp" (and NOT "secondApp://") as the name of URL Scheme, with my company as the URL identifier.

Take a look at OAuth 2.0 playground.You will get an overview of the protocol.It is basically an environment(like any app) that shows you the steps involved in the protocol.

https://developers.google.com/oauthplayground/

redirected uri is the location where the user will be redirected after successfully login to your app. for example to get access token for your app in facebook you need to subimt redirected uri which is nothing only the app Domain that your provide when you create your facebook app.

If you are using Facebook SDK, you don't need to bother yourself to enter anything for redirect URI on the app management page of facebook. Just setup a URL scheme for your iOS app. The URL scheme of your app should be a value "fbxxxxxxxxxxx" where xxxxxxxxxxx is your app id as identified on facebook. To setup URL scheme for your iOS app, go to info tab of your app settings and add URL Type.