Contextual Definition

I am looking for JavaScript examples on expanding contextual
definition. For example, you have a word like POP3 and the user clicks
on the word expanding it with the following contextual definition.

POP3 (POP3: A common protocol that is used to retrieve e-mail messages
from an Internet e-mail server.)

Sites like Microsoft use the technique throughout their site. I hope
to find the code and an explanation on its usage. Let me know if you
have any sources. Thanks!