Protocols are "interfaces" (in the general sense of the word in programming). So they are useful when you want to write a delegate (or callbacks in the general

Protocols are "interfaces" (in the general sense of the word in programming). So they are useful when you want to write a delegate (or callbacks in the general sense of the word) class. Many classes have a delegate property that is of type id<SomeProtocol>. Equivalent to Java's interfaces, but Cocoa use protocols heavily for delegates only. Categories are useful not for sharing code, but for expanding a class's functionality. This is especially useful when you write a class X that works with an existing class Y, but some of X's code should be an instance method of Y, or X introduces additional capability for Y to support X. UITableView adding functionality to NSIndexPath to support referencing table sections and rows is a good example. All those Java classes that add static methods to support existing Java classes, or if you used Smalltalk and added methods to existing classes, this is the equivalent.