Tweek is portrayed as a hyperactive boy, and has messy blond hair. He generally wears a grey-blue shirt, buttoned incorrectly, and blue jeans. The character is noted for his mannerisms; he is always nervous and jittery and usually overdosed on caffeine, leading to extreme paranoia and what his mother calls ADD. His voice is usually strained, and he tends to scream, "Aaargh! Too much pressure, I can't take it!!" or some variation thereof. Since his parents are called the Tweeks, Tweek would appear to be the character's last name. His first name has not been revealed; the other adults simply refer to him as "the Tweek boy", while the children just call him "Tweek".
Tweek's parents own a coffee shop, Tweek's Coffee, and attempt to calm him using coffee; this has the effect of increasing his caffeine levels and worsens his mental state.
Tweek himself has fought with Craig, and was chosen as the fourth friend of the main characters during a reality television spoof in which Kyle, Stan and Cartman remove Butters from their group. He was a member of the group for five episodes in the middle of the sixth season, after which he returned to his previous minor-character role. He also has an ongoing problem with underwear gnomes that steal his underwear every morning at 3:30.
(Most of this owes credit to Tweek's Wikipedia entry.)