strcpy is a unsafe function. When you try to copy a string using strcpy() to a buffer which is not large enough to contain it, it will cause a buffer overflow.
strcpy_s() is a security enhanced version of strcpy(). With strcpy_s you can specify the size of the destination buffer to avoid buffer overflows during copies.
char tuna[5]; // a buffer which holds 5 chars incluing the null character.
char salmon[] = "A string which is longer than 5 chars";
strcpy( tuna, salmon ); // This will corrupt your memory because of the buffer overflow.
strcpy_s( tuna, 5, salmon ); // strcpy_s will not write more than 5 chars.
