Scratch

XD is supposed to be a laughing face - look at it from the side. 

JK means Just Kidding.

LOL means 'laugh out loud'

XD is supposed to be a laughing face ( like a sideways > [sideways D] , ), LOL stands for 'Laugh out Loud', and is more or less just tossed around whether you do or not.  JK stands for Just Kidding.  Fun fact: ROFL is a more extreme form of LOL that is rarely used, and stands for Rolling On Floor Laughing (Artix Entertainment joked about actually trying it once- as in literally, like rolling around on their office carpet while loudly laughing.  If they actually do everything they say, then I don't envy the guys in the office next to them).  I personally have begun to think them phonetically (as in being pronounced 'La-l', instead of 'El-oh-el'), and use them in my thoughts whenever I hear or see something amusing.  Which is probably a sign of my over-computer-obssession.  I think that's why some people use 'lawl' instead of 'lol', though I don't suggest it.  I myself didn't get XD for a while until I researched it on Wikipedia and stared at it for quite some time.

i remember in 2009 i nearly made a topic asking what "xD" meant
but i worked it out luckily

i use to be really, really n00b.