Whether to use an icon on your posting, or, if you do, the icon you use, is completely up to you. If you'd like to know the common meaning of each icon or smilie, just put your cursor over one of them when you're composing a message -- a pop-up will tell you what it's supposed to connote.
Ah, yes, the acronyms -- their meaning is a little ambiguous. For example, "DH" could mean "Dear Husband", or "Dang Husband", or "Dumb Husband", or something else (but most cruisers, being nice people, prefer "Dear"). Likewise "DW" for "Dear Wife", or "DD" for "Dear Daughter", "DS" for "Dear Son", or even "DMIL" for "Dear Mother-In-Law"
In the areas dedicated to one cruise line, the ship names often get "acronym-ized", too. For example, you might see "MOS" in the Royal Caribbean areas, which can stand for "Majesty of the Seas", or "Millie" in the Celebrity areas, which means the Celebrity Millennium. It gets rather confusing in the Princess areas, since several ships' names start with the same letter, so they've standardized on things like "CB" for "Caribbean Princess."
In general, though, don't hesitate to just jump in and ask what someone means by an acronym or abbreviation they've used. Clear communication helps us all!