Chuunibyou describes teenagers around the age of fourteen that try to look cool to impress people around their age and are self-important trying to be unique.
This way of thinking, however, may continue even after the teen reaches adulthood, but it does not actually relate to any medical condition or mental disorder.
It's more like a growth phase for some teenagers.
Based on "Chuunibyou User Manual" (中二病取扱説明書, Chuunibyou Toriatsukai Setsumei Sho) by Saegami HYOUYA, there are 3 types of chuunibyou.
DQN-type pretends to be antisocial even though they aren't, or can't be and has made-up stories about antisocial behavior (gang fights/crime).
Subculture-type/Hipster prefers subculture or minor trends just to be different and have the "cool" factor. (obsessed with imitating idols/fictional characters)
Evil Eye-Type admires mystical powers and pretends to have one of their own, to the extent of establishing an alias just for that.1
An example of DQN Chuunibyou:
"Look at all those bandages wrapped around you! What happened?"
"I had a bunch of gang fights."
"Come on, I know you're too kind to be a gangster."
An example of Subculture Chuunibyou:
"When I collect Pokémon cards, I only collect ones that are imported
"Can you read all those?"
"-_-; S-sure I can..."
An example of Evil Eye Chuunibyou:
"Don't take that bandana off my left wrist. It's sealing an evil
spirit inside me and if you take it off I may go berserk."
"Dude, calm down. You just have chuunibyou."2
1 Animanga Wikia: "Chuunibyou"
2 Urban Dictionary: "Chuunibyou"
3 The Free Dictionary Language Forums: "Japanese Slang--Chuunibyou"
4 Chuunibyou demo Koi ga Shitai! :)