pointer palindrome(can someone please explain the following code step by step and in a easy way...thanks a lot!!!)

#include <stdio.h>
#include <ctype.h>
#define STR_LEN 80

int main()
{
    char s[STR_LEN+1];
    int i = 0;
    char *left;
    char *right;
    char *end;

    for (i = 0; i< = STR_LEN; i++) 
    {
        s[i] = ' ';
    }
    printf("Enter: ");
    fgets(s, STR_LEN+1, stdin);

    left = &s[0];
    end = right = &s[STR_LEN];

    while (left<right) 
    {
        while ((!((*left>=65)&&(*left<=90)))&&
               (!((*left>=97)&&(*left<=122)))&&
               (left<end)) 
        {
            left++;
        }
        while ((!((*right>=65)&&(*right<=90)))&&
               (!((*right>=97)&&(*right<=122)))&&
               (right>s)) 
        {
            right--;
        }
        if (left == end) 
        {
            break;
        }
        if (tolower(*left) != tolower(*right)) 
        {
            printf("\nNo Palindrome!\n\n");
            return 0;
        }
        left++; 
        right--;
  }

  printf("\nPalindrome!\n\n");
  return 0;
}