Classes Are Blueprints for Objects

A class describes the behavior and properties common to any particular type of object. For a string object (in Objective-C, this is an instance of the class NSString), the class offers various ways to examine and convert the internal characters that it represents. Similarly, the class used to describe a number object (NSNumber) offers functionality around an internal numeric value, such as converting that value to a different numeric type.
In the same way that multiple buildings constructed from the same blueprint are identical in structure, every instance of a class shares the same properties and behavior as all other instances of that class. Every NSString instance behaves in the same way, regardless of the internal string of characters it holds.
Any particular object is designed to be used in specific ways. You might know that a string object represents some string of characters, but you don’t need to know the exact internal mechanisms used to store those characters. You don’t know anything about the internal behavior used by the object itself to work directly with its characters, but you do need to know how you are expected to interact with the object, perhaps to ask it for specific characters or request a new object in which all the original characters are converted to uppercase.
In Objective-C, the class interface specifies exactly how a given type of object is intended to be used by other objects. In other words, it defines the public interface between instances of the class and the outside world.