What is the difference between gets() and scanf()

#include <stdio.h>

int main()
{
   char text[100];
   int length = 0;

   gets(text);

   while (text[length] != '\0')
      length++;

   printf("%d",length);
}

I was trying to count the letters in an string using the above program. it worked. But, when i tried the same program with scanf() instead of gets(), it didn`t work. pls help me...