IRC/chatterbox/email jargon meant to be pronounced quasi-phonetically (i.e. "kaythanks") and interpreted as "Okay? Thanks."
KTHX is a quick way of both asking if something is okay and thanking the person in advance. For example: "There will be no further discussion of soy-related products for the remainder of the day. KTHX."
Though not too often, people sometimes use this in spoken language as well, usually when they're trying to be silly and/or pretend as if they're cajoling folks into something. For example: "So, I was thinking you should spend the night tonight. Sound good?" ...then add very quickly kaythanks! before the person in question has a chance to answer.
Other times KTHX is used with the intent of being kind of snotty or sarcastic, like a type of "obviously fake politeness." (You see this more commonly on IRC, in email, etc., than you do in the spoken form.)
Examples of this would include: "Hey, I've got a plan! How about you shut the hell up and stop emailing me regarding your fantastic prices on herbal Viagra? KTHX;" or simply, "Hey, how about everyone of you bastards just get down on your knees and blow me? KTHX."
It can also be used as a verb, as in...oh, wait...no, no it can't.