Buffer Overflow Intro. ©2002, Jedidiah R. Crandall, Susan L. Gerhart, Jan G. Hogle.  http://sfsecurity.pr.erau.edu
Want a more detailed but non-techie explanation?
ANALOGY: computer~~mailroom
Scenario Characters:
Norman, the mailroom worker
Alice, who programs the mailroom computer
Patty, an innocent-appearing user who knows how to use a buffer overflow to hijack the computer