Prime number in C/C++

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C++

C

Here is my doubt , a friend of mine pass me this code, that the function is finding if the provided number is prime or not .
The problem is that I do not understand why the code works,get it? I can not understand the logic behind the algorithm.
Here is the code :http://www.mediafire.com/view/?c2o477ach484kbd
Thanks for any help .

int main(void)
{
int n, c = 2;

printf("enter the value:");
scanf("%d",&n);

for ( c = 2 ; c <= n - 1 ; c++ )
{
if ( n%c == 0 )
{
printf("not a prime number");
break;
}
}
if ( c == n )
printf("prime number");

system("PAUSE");	
return 0;
}