[Timothee]

It's somewhat possible though not very widely used.

Basically, any iOS app can register a URL scheme and be opened with that scheme and anything after that by another app. This is used for example by Camera+: http://api.camerapl.us/app-api

With their API, another app can launch Camera+ to edit a photo and Camera+ sends the user back to the original app with the data of the edited photo.

Facebook uses it for single-sign-on of third-party app. (i.e. user taps "sign in with Facebook", the Facebook app is launched, the user taps "Accepts" and is redirected towards the original app)

One big limitation of this mechanism is that it's very much ad-hoc.

Another existing mechanism is the one where apps can register themselves as being able to handle a certain type of documents. You can then open a document from another app.