C言語では、fork()とexec()の違いは何ですか?

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/wait.h>
int main() {
   pid_t process_id;
   int return_val = 1;
   int state;
   process_id = fork();
   if (process_id == -1) { //when process id is negative, there is an error, unable to fork
      printf("can&#39;t fork, error occured</p><p>");
         exit(EXIT_FAILURE);
   } else if (process_id == 0) { //the child process is created
      printf("The child process is (%u)</p><p>",getpid());
         char * argv_list[] = {"ls","-lart","/home",NULL};
      execv("ls",argv_list); // the execv() only return if error occured.
      exit(0);
   } else { //for the parent process
      printf("The parent process is (%u)</p><p>",getppid());
      if (waitpid(process_id, &state, 0) > 0) { //wait untill the process change its state
         if (WIFEXITED(state) && !WEXITSTATUS(state))
            printf("program is executed successfully</p><p>");
         else if (WIFEXITED(state) && WEXITSTATUS(state)) {
            if (WEXITSTATUS(state) == 127) {
               printf("Execution failed</p><p>");
            } else
               printf("program terminated with non-zero status</p><p>");
         } else
            printf("program didn&#39;t terminate normally</p><p>");
      }
      else {
         printf("waitpid() function failed</p><p>");
      }
      exit(0);
   }
   return 0;
}

The parent process is (8627)
The child process is (8756)
program is executed successfully