id type

This type defines a variable whose type is defined at runtime.

Category

Categories is used to add methods to an existing class without inheriting from it. For example, to add method <function name> to class <class to add methods>, we need to declare:

#import <Foundatiion/....h> // class we want to add methods to

@interface <class to add methods> (<name of category>)
- (<return type>) <function name>;
@end

and implement in another file:

#import <....h> // header file with declaration

@implementation <class to add methods> (<name of category>)
- (<return type>) <function name> {
  // insert code for implementation
}
@end

Portocol

This is the same as interface in other programming languages. To declare:

@protocol <name of interface>
- (<return type>) <required function name>;
- (<return type>) <required function name>;

@optional
// everything after the above directive is not required, otherwise
// the class supporting the interface must implement the first 2 methods

- (<return type>) <optional function name>;

@required
// everything after the above directive is required.
- (<return type>) <required function name>;
@end

To adopt to an interface (must implement required methods from interface1, interface2, etc.):

@interface name : parent <interface1, interface2...>

You can check at runtime if object conforms to an interface by sending a conformsToProtocol message:

if ([<object> conformsToProtocol:@protocol(<interface>)]) {
  // do something
}

When declaring a method, it is possible to require interface to be supported by parameters:

- (<return type>) <function name>: (id <interface>) param0;

Here param0 can be any type (due to id), as long as the type implements interface.