Protocol

A protocol is a declaration of a group of methods not associated with any particular class.

Formal Protocol

Formal Protocols are used to define the Interface for a set of methods, but not the implementation or any data members.

This is useful for defining an interface which objects will need to conform to in order to take advantage of some service or other.

Example:

   @protocol MyFormalProtocol
   
   -(void) someMethod;
   -(id) someOtherMethod;
   
   @end

Informal Protocol

Informal Protocols are constructs that allow you to share a common Interface between two classes without them having to inherit from any specific object (the notable exception being NSObject).

They are generally made with a category on NSObject that has no implementation associated with it.

Example:

   @interface NSObject (MyInformalProtocol)
   
   - (id) someMethod;
   - (void) anotherMethod;
   
   @end

Implementing Protocols

Formal and informal protocols take slightly different implementation paths. For formal protocols, you enclose the protocol name in less than/greater than signs e.g.:

   @interface MyClass <MyFormalProtocol>
   
   /* Methods in protocol */
   
   -(void) someMethod;
   -(id) someOtherMethod;
   
   /* Your methods */
   
   @end    

With informal protocols, you place the protocol name in brackets after your class name, both in the interface and implemenation i.e.:

   @interface MyClass (MyInformalProtocol)

and

   @implementation MyClass (MyInformalProtocol)

Although the interface declaration is optional.