I know most of this has been answered, but bleh heres a summary.
If someone says WTF* where * is any number of random words or characters. Then do not bother trying to decypher it as some sort of important phrase. They are joking, it matters not whether it says bbq, watermelon or ducknuts.
YA*T = Yet Another * Thread. This isn't really important, unless you were trying to find all the car or girl threads you could search for YACT / YAGT. Since those are the only scenarios where it is used with any sort of regularity. You will also get gun threads with girl threads...
JRun is an error that has been happening on AT since the forums were updated. Google for JRun Errors if you want to get plenty of feedback.
These 2 terms are commonly used in the body of posts, although the second one not so much around here:
TTT = To The Top
NT = No Text