Okay, there are a lot, and I don't know all of them.
Jobs
WAR - Warrior
MNK - Monk
THF - Thief
WHM - White Mage
BLM - Black Mage
RDM - Red Mage
RNG - Ranger
PLD - Paladin
DRK - Dark Knight
SAM - Samurai
NIN - Ninja
BST - Beastmaster
SMN - Summoner
BRD - Bard
DRG - Dragoon
Chat
OMW - On My Way. This is pretty useful, and I use it often
OMG - Oh My Goodness (or whatever). I find this annoying and way over-used.
LOL - If you're young "Laugh out loud"... If you're older "Lots of Laughs". Either way, same effect.
TNL - "To next level". A question, asking you (or someone else) how many experience points they need before they hit their next level.
EXP - Experience points.
BRB - "Be right back".
BBL - "Be back later".
AFK - "Away from keyboard". Means you've stepped away for a moment. Typically you do this so that your cohorts know you aren't around and they don't attack anything nasty without you. Really, this is a polite way to excuse yourself to go to the bathroom.
Enemies
MOB - Something describing enemies or other moving objects (Mobile Objects). I rarely see this one used.
AGGRO - Aggressive. Used to describe something (enemy) which will or may be very aggressive.
LINK - Linking attacks. Used to describe either attacking as a group or being attacked by a group. "MOB AGGRO LINK" could describe some very nasty enemies ;-)
NM - Notorious Monster. This is a monster that is notorious for some reason. Typically they are rare, and often they are much harder than the baddies around them.
BCNM - Burning Circle Notorious Monster. These are typically bosses you fight in missions. You walk up to certain areas which have these "burning circles" and do something to trigger the battle. The first one you will likely encounter is the Dragon in mission 2-3.
When I email my screenshots somewhere, why are they so shitty?
There's a number of reasons for this.
One, the FFXI developers chose to use the JPEG format (which is especially odd, because they obviously have PNG support in there- just look at the greeting cards!) JPEG is what is known as a "lossy" format in that it loses information for the sake of compression. So, anytime you have a JPEG image pixels, colors, and detail in general will be lost.
Two, it seems the FFXI developers wanted these JPEG images to take up as little room as possible, because the compression is very high with the quality very low. Either they did this on purpose, or they're just idiots. I prefer to think the former.
Three, the JPEGs are often at a completely different gamma than whatever your computer's display is set at. This means that they could be darker (likely) or lighter (unlikely).
Four, I haven't confirmed this, but I have suspicions they are also scaling the images down. Which will, of course, further reduce the quality.
Now, bear in mind, I'm only talking about the PS2 version here. I assume the PC version probably just stores it's screenies in BMP or some other lossless format. I could be wrong, and if so then WTF? I hate it when developers dont just dump the images in a lossless format and let me decide what the final format I want is!