Abbreviations can be a huge PITA if you don't know what they mean. Here are a few common ones to help even the least techy peeps know what's going on, and when they're appropriate to use.
The second-to-worst message to ever receive (the worst of course is, "k").
IDK: I don't know
What women say when you ask what they want for Valentine's Day (WHICH I STRONGLY CAUTION YOU NOT TO TAKE LITERALLY).
IDC: I don't care
If you asked a girl on a date via text and left it with this, her response will (or should) be, "NO."
LMK: Let me know
Probably the most annoying of all time. I hate even including it on this list, but so many people use it that I had to.
ROFL: Rolling on the floor laughing.
This should be a filter on Facebook.
TMI: Too much information
The most common one-word response to anything. I find it ironic that this response could be shortened at all, but ta-da! Thank you 14-year-old's everywhere.
B/C: Because
How my Dad refers to the truck. Also a term for Spykids.
POS: Piece of s***, or more commonly in instant messaging: parent over shoulder.
Used to insert your deviant opinion without looking like a complete a-hole.
IMHO: In my humble opinion
Use frequently, as everything said online (or not F2F) is true.
JK: Just kidding
Most often used by creepy people-watchers at the airport (yes, I openly include myself in this group).
OH: Overheard
Commonly used at the end of an email when the sender needs a quick turnaround on a project, but is afraid to ask for it.
BTW: By the way
Because you never know if the other six letters may lead to carpal tunnel...
TY: Thank you
If he didn't type it all out, by golly, why should you?
YW: You're welcome
Interchangeable with the term above in case you really want to spice things up.
NP: No problem, not a problem
Should probably be accompanied by a blonde girl emoticon (I can make that joke because I am a blonde girl). Also appropriate to use if someone breaks up with you via text message. B/C… I mean… what else would you say?
OMG: Oh my God, gosh, goodness, golly, you get it.
90s shorthand salutation, also important to know if you live, work, or talk with individuals with memory loss
CYA: C (see) ya, or also: cover your a**.
When something is funny, but not THAT funny. Never send in relation to bad news, like the woman below did.
LOL: Laugh out loud, no longer: "lots of love"

Have more to add? Share them in the comments below!