If you're familiar with C# and .NET, it's a no brainer.
Xamarin is nothing more than a C# wrapper around the native iOS and Android APIs. Everything you're familiar with (UIViewController, UIView, UITextField, UIButton, etc) remains the same. The main benefit (other than using C#) is that all non-UI code can be shared across platforms (iOS, Android, Windows, server, etc).
Xamarin.Forms is built on top of Xamarin and lets you reuse the same code for UI as well (using their own abstraction that's similar to WPF/UWP). I would only recommend this for relatively simple apps as you lose some control over platform specific details.
Xamarin is nothing more than a C# wrapper around the native iOS and Android APIs. Everything you're familiar with (UIViewController, UIView, UITextField, UIButton, etc) remains the same. The main benefit (other than using C#) is that all non-UI code can be shared across platforms (iOS, Android, Windows, server, etc).
Xamarin.Forms is built on top of Xamarin and lets you reuse the same code for UI as well (using their own abstraction that's similar to WPF/UWP). I would only recommend this for relatively simple apps as you lose some control over platform specific details.