https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/DefiningClasses/DefiningClasses.html#//apple_ref/doc/uid/TP40011210-CH3-SW18

In Objective-C, a class is itself an object with an opaque type called Class. Classes can’t have properties defined using the declaration syntax shown earlier for instances, but they can receive messages.

The typical use for a class method is as a factory method, which is an alternative to the object allocation and initialization procedure described in Objects Are Created Dynamically. The NSString class, for example, has a variety of factory methods available to create either an empty string object, or a string object initialized with specific characters, including:

+ (id)string;

+ (id)stringWithString:(NSString *)aString;

+ (id)stringWithFormat:(NSString *)format, …;

+ (id)stringWithContentsOfFile:(NSString *)path encoding:(NSStringEncoding)enc error:(NSError **)error;

+ (id)stringWithCString:(const char *)cString encoding:(NSStringEncoding)enc;

As shown in these examples, class methods are denoted by the use of a + sign, which differentiates them from instance methods using a - sign.

Class method prototypes may be included in a class interface, just like instance method prototypes. Class methods are implemented in the same way as instance methods, inside the @implementation block for the class.

Objective-C Classes Are also Objects