Difference between int main() and int main(void)?

In C++, there is no difference.

In C, the difference is questionable. Some love to argue that the latter version (the one without void) is technically just a common implementation extension and not guaranteed to work by the standard because of the wording in the standard. However, the standard clearly states that in a function definition an empty set of parameters has a well-defined behaviour: that the function does not take any parameters. Thus such a definition for main matches the following description in the standard:

It [main] shall be defined with a return type of int and with no parameters.

There is, however, a noticeable difference between the two: namely, the version without void fails to provide a correct prototype for the function:

// this is OK.int main(){  if (0) main(42);}// this requires a diagnostic to be shown during compilingint main(void){  if (0) main(42);}

Oh, and just to be complete: the void has the following meaning in all function declarators:

(6.7.6.3p10) The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.

In C, in a prototype (not in C++ though) an empty argument list means that the function could take any arguments (in the definition of a function, it means no arguments). In C++, an empty parameter list means no arguments. In C, to get no arguments, you have to use void. See this question for a better explanation.

First of all, there is a difference of what is allowed for hosted systems and freestanding systems, as shown here.

For hosted systems, 5.1.2.2.1 Program startup applies:

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

int main(void)

... (more text follows regarding argv/argc etc styles).

The interesting part is "with no parameters". int main() and int main (void) are currently equivalent, since they are both function declarators and have no parameters. The following applies (6.7.6.3 ):

10 The special case of an unnamed parameter of type void as the only item in the list specifies that the function has no parameters.

/--/

14 An identifier list declares only the identifiers of the parameters of the function. An empty list in a function declarator that is part of a definition of that function specifies that the function has no parameters. The empty list in a function declarator that is not part of a definition of that function specifies that no information about the number or types of the parameters is supplied.145)

Emphasis mine, the bold text is what applies to int main(). There is also note 145) at the end of the text, which says "See ‘‘future language directions’’ (6.11.6)":

6.11.6 Function declarators

The use of function declarators with empty parentheses (not prototype-format parameter type declarators) is an obsolescent feature.

And here is the difference. Being a function declarator, int main() is bad style because of the above, since it is not guaranteed to work in the next version of the C standard. It is flagged as an obsolescent feature in C11.

You should therefore always use int main (void) on a hosted system and never int main(), even if the two forms are, for now, equivalent.

In C++ both forms are completely equivalent, but there int main() is the preferred style for subjective, cosmetic reasons (Bjarne Stroustrup says so... which is probably quite a bad rationale for explaining why you do something in a particular way).