A stupid, pimply faced, teenager who is popular in their school, and thinks having a social life is more important than drinking water, and only knows how to say "FAIL" and/or "EPIC FAIL" as a comeback to any good insult. Fail kids like to take out their frustrations on popular online games like warcraft or counter strike, because they're nerdy, weak ass little pre-mature fucks in real life, and stay up till midnight on those games using lame internet slangs like L0l, pl0x, own3d, n00b, OMG you noob omg!, and of course, FAIL. They can be seen hanging out in malls with their friends(who are also fail kids). When these angry ass teens aren't saying fail, hanging out with friends, eating mcdonalds, or playing games, they like to go on facebook, and get depressed after random ass people dont accept their friend requests.
A kid who thinks he is funny by saying things but really isn't. This type of person generally fails at life, has no friends and sits at their computer all day long. These people are different from trolls in the sense that they fail at trolling too and have no actual value to contribute to society or life.
A fail kid is someone who stalks you on facebook, posts random fail comments and thinks hes cool and good at everything they do.