The Fission Theory: This theory proposes that the Moon was once part of the Earth and somehow separated from the Earth early in the history of the solar system.
This provides modular approach to programming. Classes. A class is a concept used in object oriented programming languages such as C++, PHP, and JAVA etc. Apart from holding data, a class is also used to hold functions. An object is an instant of a class. In case of variables, the type is the class whereas the variable is the object.
An update to the above answers, I agree instance methods use an instance of a class, whereas a class method can be used with just the class name. There is NO more any difference between instance method & class method after automatic reference counting came to existence in Objective-C.