Using several files?

When I have more than one source files do I need then declare all others files functions in main file with extern. And I can`t understand how to use the class what is made in other file. Then extern dosn`t help anymore.

Code:

---

#include "file.c"

int main(void)
{
  //main function is only in this file
  //the other functions are in file.c
  //you can use them as if they were
  //in this source file though

  return 0;
}

---

Thanks for answering but that was not what I mean.
h fails and source fails are two different things.
In you code there are h fail with name c.
I mean another file what is compiling separatly.