Summarizing:

fork
Called one time, it returns two times. It clones the calling process.
Parent and child continue their execution from the next line.
The child process receives a copy of the stack and of the heap.
Instead, the code is shared.

Prototype: pid_t fork (void)
Return values
* 0 in the child
* the pid of the child in the parent
* -1 on error

execve
The process that calls the exec is completely substituted by the new process. The new process is executed starting from the main.

v = array of pointers
e = receives the "environment variables" as parameter, instead of using the current one

Prototype: int execve (char *path, char *argv[], char *env[]);
On success the function does not return; on error it returns -1.

system
Executes a command (specified as a string) within a program.

Example 1: system ("ls -laR");
Example 2: char s[100];
           strcpy (s, "ls -laR");
           system (s);
It is implemented using fork, exec and waitpid.