Complete review of @ in Objective-C, by Mattt Thompson:
@, or “at” sign compiler directives, are as central to understanding Objective-C’s gestalt as its ancestry and underlying mechanisms. It’s the sugary glue that allows Objective-C to be such a powerful, expressive language, and yet still compile all the way down to C.
@ is at the core of Objective-C, even more with the new "literals" (like dictionary, array): when you develop on iOS, @ is everywhere, used for instance variable visibility, properties, protocols, literals etc…
I bet you have never heard of
@compatibility_alias (I haven’t…).