What's the difference between Xamarin.iOS and monotouch?

It looks like Xamarin.iOS is a newer version of monotouch, is that true?

I tried ActiveDriectory Library 3.3 alpha, it cannot add .dll files (missed Microsoft.IdentityModel.Clients.ActiveDirectory.Platform.dll) to Xamarin Forms iOS project correctly.

If I add it manullay, it will throw exception while running said cannot find IdentityModel.Clients.ActiveDirectory.Platform.dll. The library has a reference to Xamarin.iOS not monotouch.

If I add both (Xamarin.iOS and monotouch), there is an error in editor says multi-reference.