How does #include differ from #import?

Middle PRO

Essence

All three are ways to pull declarations from another file.

• #include "a.h" — the C preprocessor directive: textual insertion of the file. If a header lands twice (through a chain of includes) you get a "redefinition" error. Protected manually with include guards (#ifndef A_H / #define A_H / #endif) or #pragma once.
• #import "a.h" — the Objective-C extension: the same textual insertion, but the preprocessor remembers the file was already included and skips it the second time. No guards needed. The standard for .h/.m.
• @import Foundation; — modules (Clang modules, since Xcode 5). Not text insertion but attaching a precompiled description of the framework (module map + serialized AST). Benefits: faster compilation (the header is parsed once for all files), autolinking of the framework — no need to add it to Link Binary With Libraries, and the module's macros do not "leak" into your code. With Enable Modules on, a plain #import <Foundation/ Foundation.h> of a system framework is implicitly turned into @import.

#import <Foundation/Foundation.h>   // implicitly becomes @import Foundation;
@import UIKit;                      // explicit module, autolinking
#import "MyModel.h"                 // own header, duplicate protection
#include <stdlib.h>                 // plain C

The <...> form searches system paths and frameworks, "..." looks next to the file first.