posted on Mar, 23 2005 @ 02:07 AM
The word you're looking for is Acronym.
The example you used IMO is an acronym for In My Opinion.
The Jargon File
is a pretty decent, if textually dense, resource if English (or Internet English, for that
matter) isn't your first language.
has some common, and some out-of-date expressions and acronyms. It's specific
to science-fiction fandom, but a lot of them have moved beyond that sphere into the wider internet (or were borrowed from it in the first place).