Forward declaration is a kind of incomplete definition where compiler does know the memory layout of the declared variable but it can use this variable as a pointer since pointer size is same for all the variables. One more advantage is all the functions which are used in the program but defined somewhere else, declared first in the program.
Example:
struct X; // forward declaration
void f(struct X*) { } // usage of the declared, undefined structure
// void f(struct X) { } // ILLEGAL
// struct X x; // ILLEGAL
// int n =sizeof(struct X); // ILLEGAL
// later, or somewhere else altogether
struct X { /* ... */ };
