what is a buffer overflow

http://it.slashdot.org/article.pl?sid=05/02/28/1259211&from=rss

The most common form is as follows. When a subroutine is called the return address is placed on the stack. Then all the local variables for the subroutine are placed on the stack. the subroutine runs and when it finishes it jumps to the return address on the stack. However if the subroutine were to write data into an array or string on the stack and tried to push more data into the string than space was allocated it would continue writing past the end of the array and eventually overwrite the return address. This allows a way to substitute a new return address for a virus maker. If this return address happened to jump right back onto the string itself then in principle the data string will now be exceuted as code.
partial remedial solutions include commands that prevent decleared data from being executed, having the return address stored on a different stack from the data stack, explicitly testing the stack integrity before executing a return from a subroutine, and putting up "electric fences" --basically buffer regions around every memory allocation that are not owned by the application requesting space.