Understanding Dispatch
Developers of computer software typically organize source code into named blocks variously called subroutines, procedures, subprograms, functions, or methods. The code in the function is executed by calling it - executing a piece of code that references its name. This transfers control temporarily to the called function; when the function's execution has completed, control is typically transferred back to the instruction in the caller that follows the reference.
Function names are usually selected so as to be descriptive of the function's purpose. It is sometimes desirable to give several functions the same name, often because they perform conceptually similar tasks, but operate on different types of input data. In such cases, the name reference at the function call site is not sufficient for identifying the block of code to be executed. Instead, the number and type of the arguments to the function call are also used to select among several function implementations.
In "conventional", i.e. single-dispatch object-oriented programming languages, when invoking a method ("sending a message" in Smalltalk, "calling a member function" in C++), one of its arguments is treated specially and used to determine which of the (potentially many) methods of that name is to be applied. In many languages, the "special" argument is indicated syntactically; for example, a number of programming languages put the special argument before a dot in making a method call: special.method(other, arguments, here), so that lion.call would produce a roar, whereas sparrow.call would produce a cheep.
By contrast, in languages with multiple dispatch, the selected method is simply the one whose arguments match the number and type of the function call. There is no "special" argument that "owns" the function/method carried out in a particular call.
The Common Lisp Object System (CLOS) is an early and well-known example of multiple dispatch.
Read more about this topic: Multiple Dispatch