What is Expires

Expires is a HTTP header that is a date for which a file is not to be updated until beyond that date.

Yahoo uses Expires (http://developer.yahoo.net/blog/archives/2007/05/high_performanc_2.html) and they recommend changing the filename if you ever need to change a file that has previously had a far future date.

Generally speaking, Expires is not something that is really applicable on smaller sites.