How does a buffer overflow attack work?

What will be an ideal response?

In a buffer overflow attack, an attacker finds a vulnerability in poorly written code that doesn't check for a defined amount of memory space use. If a program defines a variable size of 64 bytes (the total amount of memory the variable is supposed to use), and the program writes data over the 64-byte mark without triggering an error or preventing this occurrence, you have a buffer overflow. The attacker writes code that overflows the buffer, which is possible because the program accepts unvalidated user input. The trick is to not fill the overflowed memory with meaningless data but to fill it with executable program code.