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A protocol declares methods that conforming classes must (or may) implement — like an interface.

@protocol Greetable <NSObject>
- (NSString *)greeting;        // required by default
@optional
- (NSString *)formalGreeting;  // marked optional
@end

@interface English : NSObject <Greetable>     // declare conformance
@end

@implementation English
- (NSString *)greeting { return @"Hello"; }
@end

The <Greetable> after : NSObject declares conformance. The compiler warns if required methods are missing.

Multiple protocols — separated by commas:

@interface MyClass : NSObject <Greetable, NSCopying, NSCoding>
@end

Protocol-typed variables:

id<Greetable> obj = [English new];
[obj greeting];

id is the dynamic Objective-C type. id<Protocol> constrains it to objects conforming to that protocol.

Checking conformance at runtime:

if ([obj conformsToProtocol:@protocol(Greetable)]) {
    [obj greeting];
}

if ([obj respondsToSelector:@selector(formalGreeting)]) {
    [obj formalGreeting];
}

@selector is a compile-time reference to a method name. Used heavily for callback registration (target/action pattern).

Delegate pattern — Cocoa's main use of protocols:

@protocol DownloadDelegate
- (void)downloadDidFinish:(NSData *)data;
- (void)downloadDidFail:(NSError *)error;
@end

// Class holds a weak reference to its delegate, calls back on events.